Abstract: The sealing washeer parts were treated through nitrocarburizing and quenching and tempering process in order to obtain high hardness and good resistant-wearing in the surface of the part and good synthetical performance in the center of the part. This parts deformed and cracked easily during usage. Microstructure and micro-hardness were analysed and compared with qualified parts. The result shows the reason why sealing washer parts destroyed is that carbon content of ST12 steel used is too low. High carbon content and strength material should be used to produce the parts.