The fatigue crack growth behavior of Mod.9Cr-1Mo steel was investigated as a function of temperature, loading frequency and R ratios in the Paris regime. The relationship between fatigue crack growth rate and stress intensity factor range was acquired for each test condition. The results revealed that crack growth rate was accelerated with increasing temperature and decreasing loading frequency. The influence of the R ratio on crack growth rate was only pronounced at the low loading frequency condition. In order to understand the crack growth mechanism, activation energy analysis and normalized Delta K analysis were performed. This study suggests that oxidation and the degradation of mechanical properties promote crack growth behavior.