Abstract: Compressive or tensile tests were conducted on crystal tuff, silicarenite, and silts-tone samples under different strain rates,and also a great amount of literature consulted,it was found that the strength of rock rmains constant when strain rate varies from 10-4 （sec.-1） to 10-1 （sec.-1）, and above 10-1 or 100 （sec.-1）, the strength will increase obviously, and the cause of which can be explained by analyzing the fracture propagation in rock specimen.According to the phenomena that the pre-failure deformation increases or decreases with strain rate, a critical strain rate conception was postulated for studying the post-failure properties of rock.Also, the strain rate sensitivity for both hard rock and soft rock was examined.It was found that the strain rate sensitivity will decrease with the increase of confi-nning pressure on rock.