A federal judge today ordered a 30-month prison term for Benjamin H. Sasway, the first American since the Vietnam war to be jailed for failing to register with the Selective Service System.

U.S. District Court Judge Gordon Thompson Jr. seemed to relax his uncompromising attitude toward the 21-year-old college student by allowing his release on $10,000 bail, pending appeal.

He also sentenced Sasway under a provision of the law that would permit Sasway's release even before he had served the usual minimum of one-third of his prison term and specified that he be sent to a minimum-security camp facility.

Sasway has been in the municipal correction center here since Aug. 26 when a jury convicted him of failing to fill out a Selective Service registration card and Thompson refused an appeal for release on bond.

Sasway is one of only two men convicted and 11 indicted for resisting the new registration law, although government officials estimate that about 500,000 men have failed to register. That number has dropped from about 674,000 since last spring.

Officials estimate that as many as 100,000 of those who have not registered are in the military reserves or National Guard, further reducing the number violating the law.

Sasway's father, Joseph, a community college theater arts teacher, told reporters after the sentencing that he was prepared for the maximum penalty of five years in prison and $10,000 fine. "Anything less than the worst is something of a relief," he said.

Before announcing the sentence, Thompson listened intently as Sasway read a statement saying he had "violated the letter of the law" and was willing to take "full responsibility for my action." Although he knew that registration did not mean military service because there is no draft under way, Sasway said, he still resisted because he felt that "registration leads to a draft and a draft leads to involvement in unjust, Vietnam sort of wars."

Thompson gave little clue to his reasons for deciding on a sentence considered surprisingly lenient by many who had watched him during the trial. He said he had consulted federal judges, prison officials and Sasway's probation officer who referred to Sasway in his report as "a young man of high moral standards whose offense was dictated by his conscience."

Thompson told Sasway's attorney, Charles T. Bumer, that he thought the grounds for appealing Sasway's conviction are "frivolous" but added that the 9th U.S. Circuit Court of Appeals might not agree. He released Sasway after Bumer presented a plan guaranteeing that Sasway will remain in the area and appear at further hearings or surrender himself if he loses an appeal.

Sasway's grounds for appeal include arguments that the Selective Service Act is unconstitutional, that he could not be convicted because he had no criminal intent and that prosecutors illegally singled out him and other outspoken resisters for prosecution.

Sasway wrote a letter to President Carter and several newspapers in 1980 and has since appeared often in public to announce his refusal to register after Congress passed the registration law to show U.S. resolve in the face of the Soviet invasion of Afghanistan.

A federal judge in Los Angeles ruled preliminarily last week that registration resister David Wayte, 21, had been illegally singled out for prosecution. A hearing is scheduled Thursday for government counter arguments before a final decision.

Enten Eller, a 20-year-old college student in Roanoke, Va., is the other man convicted. He received three years' probation with an order that he register by mid-November, but he has said he will not do so.