The unmasking of U.S. double agent Dung Krall has thrust the Vietnamese-born woman into a world of fear, threats, harassment and attention she never thought possible when the Central Intelligence Agency enlisted her as an espionage agent three years ago.

Since her identity became known in March, former CIA operative Krall has publicly testified about her life and her job, has had her suburban Virginia home vandalized, and has faced numerous anonymous threats to herself and her family.

According to some officials, the government decided to run that risk for the opportunity to accuse a former U.S. Information Agency officer and a Vietnamese national of passing classified State Department cables through Krall to Vietnamese diplomats in Paris.

It may also explain why government have been extremely relucant in the past to expose any of their covert agents and why Justice Department officials have placed pressure on government prosecutors to win the current case. "If we lose this case," conceded one Justice Department lawyer, "it's going to be very hard ever getting other government informants to testify."

For Krall, 32 the daughter of a Vietcong diplomat, the decision to drop her secret identity and her code name "Keyseat", was one she and some top government officials agonized over for five months. Like many of the decisions an undercover agent makes, it may have been dictated, in part, by factors beyond her control.

So, too, has been the reaction to her unmasking. A month ago, someone broke into her Springfield townhouse, shattered a ceiling light, and left a single word scrawled in Vietnamese on a note pad. The word, according to friends, was "traitor."

Sources close to Krall also say that since her identity was made public in March, she and every member of her family have been threatened. Some threats have also been made against Krall by anonymous callers to a Washington Post reporter who has covered the espionage case.

One woman caller said that if one of the two men accused in the case is convicted, "we take her (Krall's) son.

"We," the caller said, represent "the people of Vietnam."

Within a week the same reporter received a letter postmarked April 3. In precise handwriting, the letter said, "You spent $11,800 to hide Dung Krall's family. We'll spend manpower and time to find them. We know she has 4 sisters. Mother. Husband. Son. We'll find them . . ."

The letter was signed, "People of Vietnam."

Since then, sources have reported that Krall has received other threatening notes and postcards. One of her sisters who also lives in the Washington area was sent a postcard in which the writer threatened to burn down the sister's house. The picture on the card, sources said, was the Tomb of the Unknown Soldier in Arlington Cemetery.

Defense lawyers said they and their clients had no knowledge of any of these until they were informed of them by the government.

Krall and her husband, a Navy aviator, have moved from their Springfield home to an undisclosed location, and their 8-year-old son, Lance, has been sent out of the area, friends say. The government, in disclosing her identity, promised to keep her on the CIA's payroll at $1,200 a month for the rest of the year and to relocate her and her family.

What happens to her beyond then is uncertain, and government spokesmen decline to say how much protection, if any, the Kralls will be given. But what has happened to her in the past three years, as disclosed in the trial and in interviews with her friends, offers a rare glimpse into the world of a double agent.

Although Krall's Vietnamese father was a dedicated Vietnamese Communist. Krall was embittered by the war that ravaged her homeland, and she came to hate the Vietcong. Friends say that she would tell them that her father, the Vietcong's ambassador to the Soviet Union, was "dead" rather than recognize his role with the insurgents.

A brother was killed while training to be a helicopter pilot for the South Vietnamese army. Krall and her three sisters came to the United States during the mid-1960s. Her two remaining sisters and their mother, tran Thi Pham, came just a few days before the fall of Saigon in 1975.

It was then, Krall testified, that she began working for the CIA, first as an unpaid undercover agent. In August 1975, according to her court testimony, she visited her father in Hiroshima, Japan, where he was attending memorial services for those killed by the atomic bomb.

She took her young son with her to meet his grandfather. It was soom after that encounter that her father, in effect, introduced her to Vietnamese officials who would later be named as unindicted coconspirators in the current spy case. But Krall never told her father, whom she had not seen for 20 years, of her affiliation with the CIA.

Defense lawyers for David Truong, one of the men on trial, have charged that money was her primary motive for joining the CIA. She has denied this claim.

In the summer of 1976, she began to receive cash payments of $700 a month, and a year later - after considerable discussion, according to court testimony - she was given a raise, to $1,200 a month.

Shortly before her identity was revealed. Krall made one furtive flight to London with her mother in an effort to persuade her father to break with the Communists. He refused, and a month later she agreed to testify in the current case.

Her decision, as revealed in her testimony, was to acquiesce to pressures that began the summer before - when the Justice Department asked the CIA to let her appear in court. She had been able to refuse - to hold out for protection and a continuing government job - until the third week in January, when an FBI agent informed her that her contacts with the CIA had come to an end. contacts with the CIA had come to an end.