Abstract: The samples of granite and metamorphic-sediment rock have been obtained at three spots with the water depth of 2800m-4000m in Xinan Sea-basin of the South China Sea, where the crust thickness is about 8-10 kilometres calculated by gravitational and magnetic data and taken for oceanic crust by some tectonic geologists. These samples were dragged by Guangzhou Marine Geological Survey when they were doing the project of offshore mapping with 1:1000000 scale there. The three samples are respectively fine granitite, quartz metamorphic-sediment and granodioite by microstructure analysis. The isotopic age of the fine granitite is 109.7Ma and 114.2Ma measured by Ar-Ar and K-Ar methods and the zircon age is 120Ma, by ion microprobe. These realities are made sure that the granites formed in the late of early Cretaceous, and therefore they belong to a part of Mesozoic granite zone of the South China quite possibly. The discovery above strongly challenges the idea that the crust of Xinan Sea-basin in the South China Sea is oceanic crust due to seafloor spreading, which is considered, up to the present.