Before reading the sentence, U.S. District Judge Cormac J. Carney said he didn't know exactly what information Chung had passed to China over a 30-year period. But just taking the "treasure trove of documents" from Boeing Co., a key military contractor, constituted a serious crime, he said.
"What I do know is what he did, and what he did pass, hurt our national security and it hurt Boeing," the judge said.
Chung asked the judge for a lenient sentence, saying he had brought home some 300,000 pages of sensitive papers to write a book.
"Your honor, I am not a spy, I am only an ordinary man," said Chung, who wore a tan prison jumpsuit with his hands cuffed to a belly chain as his wife and son watched from the audience. "Your honor, I love this country. ... Your honor, I beg your pardon and let me live with my family peacefully."
During sentencing, Assistant U.S. Attorney Greg Staples noted Chung amassed a personal fortune of more than $3 million while betraying his adopted country.
Chung's lawyers argued during the trial, he may have violated Boeing policy by taking home the documents, but did not break any laws. But the judge said evidence showed Chung was passing information to Chinese officials as a spy.
Prosecutors sought a 20-year sentence to send a message to other would-be spies. The judge, noting Chung's age and frail health, sentenced him to 15 years and eight months.
Investigators discovered Chung's activities while investigating another suspected Chinese spy living and working in Southern California.
Chi Mak was convicted in 2007 of conspiracy to export U.S. defense technology to China. He was sentenced to 24 years in prison.
The Associated Press contributed to this report.