Abstract:Fatigue crack growth rates of alloy 690 have been generated in air at room temperature (RT)，in air at 325℃ and in deoxidized ultrapure water at 325℃ based on direct current potential drop (DCPD) method. The results have been analyzed by Priddle model, thus the threshold of stress intensity factor amplitude (△Kth) and the fracture toughness (Kc) were obtained in these three conditions. Results showed that, fatigue crack growth rates were accelerated in water environment, which could be explained by slip dissolution mechanism. In higher temperature, fatigue crack growth rates were also accelerated because material strength was weakened which lead to decreasing in △Kth and Kc. Scanning electron microscopy (SEM) examination of the fatigue fracture surfaces revealed similar slipping ledges with transgranular cracks for specimens in air, and different crack surface with both intergranular and transgranular cracks in water environment.