Diamond thin films were synthesized on WC-Co substrate by RF PACVD(radio frequency plasma-assisted chemical vapor deposition) technique with H$_2$-CH$_4$-O$_2$ gas mixture. WC-Co substrate was pre-treated in HNO$_3$solution, scratched with 3$mu extrm{m}$ diamond paste and exposed in the O$_2$ plasma before deposition. The diamond thin film prepared at 11% oxygen concentration showed the best quality of good adhesion and wear resistance at various oxygen concentration with the fixed 5% CH$_4$ concentration.