A Chinese-born engineer found guilty last year of conspiring to export sensitive US defence technology to China has been jailed for 24.5 years.
Mak had said he did not realise making copies was illegal
The court in California had also convicted Chi Mak, 67, of acting as a foreign agent and of making false statements to federal agents.
Mak had worked on submarine propulsion systems for defence contractor Power Paragon in Anaheim.
The FBI found defence CDs in luggage of Mak's relatives at Los Angeles airport.
Prosecutors had alleged that Mak gave thousands of documents to his brother, who handed them to Chinese authorities.
Four of Chi Mak's relatives pleaded guilty to related offences last year.
Hong Kong flight
Mak, a naturalised US citizen, said during last year's six-week trial that he copied sensitive documents from Power Paragon and kept copies in his office.
He said he had not realised at the time that making copies was illegal.
He was arrested in October 2005 in Los Angeles after FBI agents stopped his brother and sister-in-law as they boarded a flight to Hong Kong.
Investigators said they found three encrypted CDs in their luggage that contained documents including one on a propulsion system that could make submarines virtually undetectable.
Mak had pleaded not guilty to all charges. He was not formally charged with espionage as the information was not officially deemed to be classified.
US District Judge Cormac Carney also fined him $50,000 (£25,000).
"We will never know the full extent of the damage that Mr Mak has done to our national security," he wrote in a sentencing statement.
"A high-end... sentence will provide a strong deterrent to the PRC [People's Republic of China] not to send its agents here to steal American military secrets," the judge added.