Diamond thin films were deposited on Si substrate using $CH_4 and H_2$mixed gas by RF plasma CVD. Prior to deposition, the substrate surface was mechanically scratched with the diamond paste of $3{mu}m$ to improve the density of nucleation sites. The microstructure of diamond films deposited with methane(0.5%~2%) at the reaction pressure ranging from 20 torr to 50torrr were studied by a scanning electron microscope. It was observed in the deposited diamond films that the nucleation density decreased and crystallinity increased with decreasing the methane concentration. However, the nucleation density and crystallinity were decreased with decreasing the process pressure.