In heat treatment process, parts should be cooled uniformly in order to reduce distortion with obtaining target hardness. But it is too difficult for parts to be cooled uniformly due to vapor blanket step in oil quenching. So we investigated that conditions of heat treatment, such as temperature and stirring speed of quenching oil, wobbling of parts, and degree of surface oxidation of work, influence to cooling behavior. In addition, it was organized that the relationships between vapor blanket stage length, initiation temperature of convection stage and uneven cooling of parts. In conclusion, the quench oil which has short vapor blanket stage length intends to inhibited uneven cooling and reduced distortion of ring specimen.