The Nansha microblock in South China Sea has long been regarded as part of South China block, but no any petrologic evidence supports this idea. Using LAICPMS, this study firstly reports magmatic zircon ages for granitic rocks from the Nansha microblock, South China Sea. The ages for two plagiogranite samples from sampling location S0818 are 159.1±1.6Ma and 157.8±1.0Ma, and those for two adamellite samples from sampling location S0832 are 153.6±0.3Ma and 127.2±0.2Ma. The dating results suggest that these granitic rocks are the products of late Jurassic to early Cretaceous tectonothermal events during the Yanshanian era, the ages for 153 to 159Ma are comparable to those of the Yanshanian ganite in the Nanling region, and that for 127Ma is comparable to those of the Yanshanian ganite in the ZhejiangFujian coastal region. Of them, a relic zircon core with an age of 656.7Ma combined with petrologic data of for the Precambrian crystalline basement in the ZhongshaXisha microblocks, suggest that these microblocks distributed within the South China Sea are likely to be the Precambrian basement. Mesozoic granite in these microblocks may be the product of crustal remelting of the old Precambrian basement. The new data obtained in this study is of great significance in studying impact of Yanshanian magmatism on southern China, tectonic evolution of the South China Sea since late Mesozoic era, and rifting dynamics of microblocks in the South China Sea.