At Catholic University yesterday, Tam-Anh Chu, 21 graduated with the highest average in engineering - 3.95 out of a possible 4.0 for straight-A grades. He had only one B in four years of undergraduate study.

His brother, Nhi-Anh, 22, also graduated summa cum laude in electrical engineering with a grade-point average of 3.87.

Then younger sister, Tu-Anh 20, received her degree magna cum laude in biochemistry. Another sister, Nhat-Anh, 23, graduated with a straight-B average in nursing.

The four graduates, smiling and self-assured yesterday, are Vietnamese refugees who left their country in 1975 during the tormented two months before the fall of the Saigon government. They live with their parents, both of whom are former high school principals in a crowded apartment in South Arlington.

When we came to this country, we had no money," said his father. Ba-Anh Chu, who now works in the drafting department of the Washington Gas Light Company. "At least our family was together . . . That was very important."

Yesterday, officials said that they believe the Chus are the first family ever to have four children graduate in the same year from Catholic University and that no other large family had ever done so well academically.

"It's a most extrodinary occurrence," said Provost C. Joseph Nuesse, who has been at Catholic University for 34 years. "The Chu family is certainly a most extrordinary family."

But winning honors together is nothing new for the Chus.

In 1974, all four had honor grades in South Vietnam's national baccalaureate exam for high school graduates. Tu-Anh, then only 15, was the youngest student ever to take the examinations. She scored among the top 1 percent. Her brother, Tam-Anh, did even better, receiving the highest mark in the Dalat section of the country.

The next March and early April after a college team in Saigon, all four Chu children left Vietnam for the United States on student visas.

Their parents left on April 27, 1975, just three days before South Vietnam surrendered to Communist forces.

Even though the family had lived well in Dalat, a resort city about 100 miles north of Saigon, they said they came to the United States with only a few suitcases of clothes, books, and family mementos.

The children had student visas to attend Frostburg State College in western Maryland. The four Chus applied to Catholic University when they read in a newspaper article that it was offering scholarships to Vietnamese refugees. Catholic University gave full four-year scholarships to all four of them.

Ba-Anh Chu said his children had attended a French elementary school run by Catholic nuns. When his oldest child, Nhat-Anh, was about to enter seventh grade in a school headed by his wife, he said the family decided that all the children should go together. Nhi-Anh skipped one grade, Tam-Anh skipped two grades, and Tu-Anh skipped three grades.

"They were all ready," he said. "Why shouldn't they go ahead together?"

During their four years at Catholic University the four drove together to the campus almost every day in a series of used cars. Both sons majored in engineering. They will be going together next fall to the Massachusetts Institute of Technology, where both will be working for doctoral degrees and both have teaching fellowships.

Tu-Anh said she majored in biochemistry because she has wanted to ba a doctor since seventh grade. Next fall she plans to be a first-year student at The Medical College of Virginia, in Richmond, though how to pay for it, she said, is a major worry.

Nhat-Anh, who majored in nursing, will be going to work as a nurse, probably at Children's Hospital in Washington.

"They're good, very good," said John D. Sherman, an assistant professor of engineering. "They're serious, but they're personable, too, and they get along with their classmates . . . They seem to be fitting into the American Way of life very well."

As Nhi-Anh explains it, though, the adjustment didn't come easily and still isn't complete, and the family doesn't want to be absorbed completely by American culture.

Even thought the Chuas had studied English for six years in Vietnam, Nhi-Anh said they had difficulty with spoken English when they came to the United States.

Also, he said, "We're kind of shy, and we don't go out for things very much." For about a year, he said, few American students spoke with them, and the Chus kept mostly to themselves or socialized with the 33 other Vietnamese enrolled at Catholic University.

"I think that's the typical American attitude," he said. "to play things down and go about you own business."

During the past two years, as they have been in smaller advanced classes, the Chus said they have gotten to know their American classmates better, though their closest friends are still Vietnamese.

"I really enjoyed the nursing school," said Nhat-Anh Chu. "Some of the RNs and the other students have been beautiful to me. They've helped me a lot."

But she added, "Even though we are becoming U.S. citizens, we cannot forget about our own country."

Most of the time, Nhat-Anh said, she wears American clothes, but sometimes she said she went to class in the ao dai, the long, traditional Vietnamese dress.

"We want to keep the best of our country," she said. "We still are Vietnamese."

Nhi-Anh said the most important thing they have kept is their family which lives together in a three-bed-room garden apartment.

Materially, the people are more better off here than they are in Vietnam," he said, "But our families are more important. That we are living together as a family, we like that very much."

Besides going to university together, the Chu children have a family band, which started playing together in Vietnam. It is now performs occassionally at Vietnamese community functions in Northern Virginia.

Three of the Chus - all except Nhat-Anh - have black belts in karate. The two sons play Ping-Pong, and Nhi-Anh blamed the sport for the few Bs he got. "I fooled around with it too much," he said.

Their father, Ba-Anh, said he now earns about $14,600 a year at the gas company. Their mother, who had been a poet in Vietnam as well as a school principal, stays at home. "I want to do that," she said. "I want to help my family."

During the summer, the three oldest children have held several jobs, ranging from work as computer programmers to selling shoes and being an orderly at St. Elizabeths Hospital.

Tu-Anh, the youngest, said she worked a little, but spent most of her summers studying.

In a few weeks Nhat-Anh will start work as a nurse, her father said, mainly to help her brothers and sister get more education.

"Most of the money I earn will go for the others," Nhat-Anh said. "I think they need it."