Abstract: Satisfactory comprehensive mechanical properties of the P110 oil casing depends on proper quenching and tempering treatment conducted according to appropriate heat treatment procedure as set up on the basis of the phase change law. Therefore precise control of the phase structure will essentially ensure the product quality. With employment of the heat expansion method and the Gleeble-1500 thermal simulator as well,the curves of continuous cooling transformation(CCT) of the 25MnV steel is measured with a cooling velocity of 1~50 ℃/s,and the influence by the cooling velocity on phase change law and microstructure evolution was analyzed.