Abstract: By dilatometric test,microstructure analysis and hardness measurement,the expansion curves of continuous cooling transformation at different cooling rates were determined for U75V rail steel using a hot simulation testing machine,and the continuous cooling transformation curve was obtained.The influence of cooling rate on microstructure and hardness was studied.The results show that the product of austenitie transformation was pearlite when the cooling rate was lower than 10 ℃·s-1.Troostite and martensite were obtained at the cooling rate of 10 ℃·s-1.Only martensite was got if the cooling rate was in the range of 10-50 ℃·s-1.The hardness of the steel increased with the increase of cooling rate.