WC-C0 base cemented carbides were plasma nitrided to obtain more hadened surface layer. The surface hardness of Hv1338 which is higher than a non-treated ane by 30%, and the hardened layer thickness of about 50$mu extrm{m}$ were obtained by the treatment of 3hrs under the conditions of $550^{circ}C$, and 5torr of gas pressure of which composition was 1:1 of $N_2:H_2$. The nitrided surfaces has WIN and W2N phases in the non-coated samples and AIN phase in the alumina coated sampled as the results of X-ray results, and showed surface roughnness of 5$mu extrm{m}$ which were caused by the sputtering action of the plasma gasee. The hardenened layer exihibits an enhanced wear resistance the cuttability test.