Abstract: 28 towing samples for plankton were collected from the upper waters of the Nansha Islands and its adjacent sea areas in the spring of 2002 and 2004 for understanding the distribution pattern of modern radiolarians and their annual changes in the upper waters of Nansha sea area before and after the summer monsoon onset. The results showed that the dominant species in the spring of 2002 and 2004 were consisting of the tropical-subtropical warm species; however there were obviously annual differences in abundance distributions of radiolarians and their typical species. Noticeably, the radiolarian abundance and its dominant species were significantly different before and after the summer monsoon onset, suggesting that there was a response of radiolarians in upper waters to summer monsoon onset in the South China Sea; The radiolarian abundance after the summer monsoon onset was significantly higher than that before the summer monsoon onset, indicating that the marine environment after the summer monsoon onset would be good for the survive of radiolarians. However, it is necessary to further study the mechanism of the change and try to find more direct environmental parameters to indicate the changes in distribution pattern before and after the summer monsoon onset. This study also confirmed that T. octacantha and S. glacialis could be proxies for upwelling processes in the tropic.