Based on the isotopic chronologic results of Cenozoic alkali basalts from the South China Sea, the characteristics of volcanic activity of the South China Sea after spreading were studied. The potassium-argon ages of eight alkali basalt samples from the South China Sea, and the argon-argon ages of two samples among them are reported. Apparent ages of the whole rock are 3.80 to 7.91 Ma with an average value of 5.43 Ma (potassium-argon, whole rock), and there is little difference among samples at the same location, e.g., 4.76～5.78 Ma for location S04-12. The argon-argon ages for the two samples are 6.06 and 4.71 Ma, which lie within the age scope of potassium-argon method. The dating results indicate that rock-forming age is from late Miocene to Pliocene, which is consistent with erupting event for alkali basalts from adjacent regions of the South China Sea. Volcanic activities occur after the cessation of spreading of the South China Sea, which are controlled by lithospheric fault and the spreading center formed during the spreading period of the South China Sea. These dating results, combined with geochemical characteristics of these basalts, the published chronological data for the South China Sea and its adjacent regions, and the updated geophysical data near Hainan Island, suggest that after the cessation of spreading of the South China Sea, there occur widely distributing magmatic activities which primarily is alkali basalt, and the volcanic activity continues to Quaternary. The activity may be relative to Hainan mantle plume originated from core/mantle boundary. Based on the isotopic chronologic results of Cenozoic alkali basalts from the South China Sea, the characteristics of volcanic activity of the South China Sea after spreading were studied. The potassium-argon ages of eight alkali basalt samples from the South China Sea, and the argon -argon ages of two samples among them are reported. Apparent ages of the whole rock are 3.80 to 7.91 Ma with an average value of 5.43 Ma (potassium-argon, whole rock), and there is little difference among samples at the same location, eg, 4.76-5.78 Ma for location S04-12. The argon-argon ages for the two samples are 6.06 and 4.71 Ma, which lie within the age scope of potassium-argon method. The dating results indicate that rock-forming age is from late Miocene to Pliocene, which is consistent with erupting event for alkali basalts from adjacent regions of the South China Sea. Volcanic activities occur after the cessation of spreading of the South China Sea, which are controlled by lithospheric fault and the spreading center formed during the spreading period of the South China Sea. These dating results, combined with geochemical characteristics of these basalts, the published chronological data for the South China Sea and its adjacent regions, and the updated geophysical data near Hainan Island, suggest that after the cessation of spreading of the South China Sea, there exists widely distributed magmatic activities which was is alkali basalt, and the volcanic activity continues to Quaternary. The activity may be relative to Hainan mantle plume originated from core / mantle boundary.