The father of a Chinese man who died of bird flu has also been infected with the virus, raising fear of human-to-human transmission.
Most people killed by the disease so far have been infected by birds
The World Health Organization said it could not rule out the possibility that the H5N1 virus had spread from the son to the father.
For the most part, humans have caught the virus from sick birds.
Scientists fear if the virus gains the ability to pass easily between humans, tens of millions of people could die.
A statement on the Chinese ministry's website said a 52-year-old man in eastern Jiangsu province identified only by his surname, Lu, has been confirmed with the virus, just days after his 24-year-old son succumbed to it on 2 December.
WHO spokesman John Rainford said: "We are concerned. The fact that we have two cases here without necessarily a clear source of animal infection and within the same family means we need to make sure we do a thorough investigation."
The father began presenting symptoms on Monday and was confirmed as having the virus on Wednesday. He is being monitored in hospital.
It is also possible that both men were infected by the same source, or that they were infected separately from different sources.
Since the H5N1 virus emerged in South East Asia in late 2003, it has claimed some 200 lives around the world. Indonesia has been hardest hit, with more than 90 deaths.
Of the 26 cases confirmed to date in China, 17 have been fatal.
Suspected human-to-human transmission of H5N1 has been reported in Hong Kong, Vietnam and Indonesia, but none of the cases have been proven.