The revelation from Kim Dong-yeon, a minister in the prime minister's office, came amid concern about the country's nuclear reactors in the wake of the Fukushima disaster in Japan, Yonhap news agency reported.
Kim said an investigation of 22,000 test results of parts in 20 active facilities found 277 documents, or 1.2 percent, were fabricated.
In another investigation of 218,000 test results for five reactors under construction and three that were suspended, Kim said 2,010 documents were found to be falsified.
He said the government is working to re-examine or replace parts whose test results were forged.
Some 100 individuals from private and state-run firms have been indicted in the scandal, Kim said.
Nuclear energy provides about 30 percent of South Korea's electricity.