Abstract: The austenite continuous cooling transformation (CCT) curves of 15CrNiMn steel for drilling bit were determined by DIL805L dilatometer. The phase transformation, microstructure and hardness of the tested steel at different cooling rates were studied. The results show that the bainite structure was obtained in the tested steel over a large range of cooling rates, and the transformation zones of ferrite or pearlite and bainite were separated completely. With the cooling rate increasing, the microstructure of the tested steel became finer and the hardness increased obviously.