The Justice Department yesterday ordered a federal grand jury in Detroit to investigate the fatal baseball-bat beating of a Chinese American for which his two assailants were put on probation for three years and fined $3,780.

The sentence had caused an uproar among civil liberties and Chinese-American groups in Detroit, and the victim's mother recently met with Assistant Attorney General William Bradford Reynolds to appeal for a more thorough investigation.

On June 19, 1982, two days before he was to have been married, Vincent Chin, 27, a draftsman, was attending a bachelor party with three friends at the Fancy Pants Tavern in Highland Park, a neighborhood in Detroit.

According to published accounts, another man in the tavern began making comments about Japanese cars costing American auto workers their jobs. A scuffle followed, and a second man joined in against Chin. A tavern employe asked the men to go outside, where the fight continued.

Later that night, the two men allegedly saw Chin at a fast-food restaurant, took a baseball bat from their car and went after him.

Witnesses said that the two men chased Chin for two blocks and that, after catching up with him, one of them held Chin down while the other hit him repeatedly with the baseball bat. Chin died four days later.

Local prosecutors originally charged the two men, Ronald Ebens, 43, an auto plant foreman, and his stepson, Michael Nitz, 23, with second-degree murder. The two men later entered into a plea bargaining agreement and avoided trial by pleading guilty to manslaughter charges.

Wayne County Judge Charles Kaufman sentenced them in March to three years probation and fined them $3,780. In Michigan, conviction of second-degree murder carries a maximum penalty of life in prison and manslaughter a maximum sentence of 15 years in prison.

In putting the two men on probation, the judge said Chin had thrown the first punch. He said the defendants were employed, had no criminal records and "weren't the kind of people you send to jail."

"We're talking here about a man who's held down a responsible job with the same company for 17 or 18 years and his son, who is employed and a part-time student," Kaufman said. "These men are not going to go out and harm somebody else. I just didn't think that putting them in prison would do any good for them or society."

His action triggered marches, rallies, picket lines outside the courthouse and protests from civil rights and Chinese-American groups.

In April, under orders from the Justice Department, the FBI began investigating Chin's death for possible violations of federal civil rights laws.

Kaufman, in response to petitions protesting the sentences, agreed to review them. But in early June he decided to uphold his earlier decision.

Several weeks ago Lily Chin, the victim's mother, and representatives of Chinese-American groups met for an hour with Reynolds, head of the Justice Department's Civil Rights Division.

Reynolds told them that he considered the fatal beating to be "a terribly brutal incident that was treated with apparently way too light a sentence."

Reynolds also told them it was a valid case for dual state-federal prosecution because of the civil rights implications. Under federal law the government can charge the men with violating Chin's civil rights even though they have been convicted in a state court.

Yesterday Reynolds said that, "based on the evidence developed by the FBI," a federal grand jury will begin to consider the case Sept. 7.