Abstract: The rate of fatigue crack growth (FCG) under rotative bend loading and three-point bend loading for 45Cr steel tempered at four different temperatures (200℃, 400℃, 500℃ and 600℃)was investigated respectively. The mechanism of FCG under the above loading conditions.have been studied by. Optical microscope and electron microscopy. The experamental results show that under rotative bend loading the rate of FCG of the steelas tempered at 500℃temperature is the lowest among the rate of FCG at four different temperature But under three-point bend loading the rate of FCG decrease monotonically with the increase of the tempering temperature. These results can be explained by the different mechanism of FCG under different tempering temperatures. It is suggested that the inflence of the Ioading condition on the FCG must be noticed.