Surface layer of tantalum produced in the mixture of hydrogen and nitrogen has been investigated using SEM and XRD. It was found that the reactions on surface of tantalum in H-O-N atmosphere can be changed by plasma. The reaction products are Ta6N2.57 and/or amorphous phase instead of Ta205. Meanwhile, the roughness of surface of tantalum treated in plasamn decreases by an order, as compared with that of tantalum treated without plasma. The reason is that changed media and the interaction between tantalum and the cathode sheath of plasma prevent from forming the rough Ta205.