When he was at George Mason University last year, Kirk Kordeleski was a big man on campus.

He was treasurer of the student government, counsel for the student honor committee, and a member of the interfraternity council.

"Kirk was one of those gung-ho people," said Joe Gangi Jr., chairman of the honor committee. "He was really outspoken and concerned."

More recently the 22-year-old Kordeleski held a good job at the World Bank's credit union where he worked on a computer system that keeps track of about $70 million in deposits.

"He was an excellent employe, a very intelligent person," said yalvara Proenza, manager of the credit union where Kordeleski had worked part time before leaving school. "I've always thought he was a very straight fellow."

But early this week the George Mason student newspaper, The Boardside, disclosed another part of Kordeleski's life. Last year he had diverted about $3,175 in student government funds to himself and his fraternity, the paper reported. He had repaid all the money by last June, but was suspended from the Northern Virginia school until January 1981.

University officials confirmed the story. So did Kordeleski, saying in an interview yesterday that he wanted to make a clean breast of things and "move ahead" with his life.

After the story was printed, Kordeleski said he told credit union officials what had happened at the school and offered to quit his $12,500-a-year job there.

Proenza and Kordeleski left yesterday, and the credit union was investigating his records "as a precaution," even though it had no reason from his conduct at work to suspect any wrongdoing.

"I never have done anything like this in my life," Kordeleski said yesterday. "But [last year at George Mason] there were so many things going on at one time -- so many organizations, so many other problems. I was in a stupor. It was a very bad part of my life . . .

"I took the money with the intention of putting it back quickly," he continued. "But I didn't, and then I tried to cover it up . . . I did what was easiest, not what was right. It's a mistake I made, and I let it get away from me."

According to Kenneth E. Kelly, coordinator of student activities at the university, the money diverted included two checks totaling $500 that Kordeleski wrote to himself. Kelly said Kordeleski also wrote $775 in unauthorized checks to his fraternity, Kappa Sigma, for which he also served as treasurer. From February 1979 until June, Kelly said, Kordeleski kept about $1,800 in movie receipts instead of depositing them in the student government bank account.

The money given to the fraternity was paid back within two weeks last April, Kelly said. But Kelly said the movie receipts were not deposited until June after he suspected the funds were missing and was trying to arrange and interview with Kordeleski.

Kelly said the $500 was repaid June 27, the day after the interview in which Kordeleski acknowledged diverting the funds. Kordeleski said he had tried to cover up the transaction by making a false entry in the student government account books.

"That made me more stupid or guilty," he said. "At first I just planned to use the money that day. And then I didn't replace it the next day and the next. Then I tried to forget about it."

Kelly said the university notified the Fairfax commonwealth's attorney's office about the case, but no criminal charges were filed.

"Our [financial] control procedures were not as tight as they needed to be," Kelly said. "Now we've made some changes."

Yesterday, Kordeleski said he needed the $500 he took to pay some credit card bills. But he said he didn't remember why he had written the checks to his fraternity or why he had not deposited the movie reciepts.

"I didn't use the movie money," he said. "I just kept it locked in a drawer and in a file cabinet. I was confused. I wouldn't do it again."

Kordeleski said that "after I found out the article was going to be in the student newspaper I've hardly slept and I've hardly eaten. Now that my job's fallen through, I have no idea what I can do. Maybe I should take a small vacation and think a few things through."