Armed North Koreans seized the boat in waters between the two countries on May 6 and demanded 600,000 yuan (USD 98,000) for the men's release, its owner Yu Xuejun told AFP, adding they were probably from the Pyongyang's military.
The incident is the latest strain in the relationship between the neighbours.
Beijing is Pyongyang's sole major ally and its key provider of aid and trade, but China said it "firmly opposed" the nuclear-armed North's atomic test in February.
"We have through the relevant channels called on the DPRK to properly deal with this case, and protect the safety and legal rights of the detained fishermen," he added.
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The state-run Global Times today quoted Jin Qiangyi, director of the Asian Studies Centre at Yanbian University, as saying it was "possible that the nuclear state is taking revenge on China" after the UN imposed sanctions over the test.
Beijing's "low-key" approach to such incidents in the past "has been taken advantage of by North Korea to infringe upon Chinese fishermen's interests", he added.
Those sailors were returned without ransom after the foreign ministry said it had contacted North Korea in an effort to resolve the case, the state-run Xinhua news agency reported.
Yu had reported the latest incident to Chinese authorities, he said, but later posted details on the Internet out of frustration over an apparent lack of official action.
Xinhua yesterday quoted an official at the Chinese embassy in Pyongyang as saying that Beijing had called on North Korea to "release the boat and the fishermen as soon as possible".