API 5L X65 is commonly used in various offshore projects for risers and flow lines in various environments ranging from sweet to sour service. Corrosion fatigue crack growth rate (FCGR) and fracture toughness have been investigated in a series of sweet and sour environments. This paper focused on the gaps that were not addressed in other studies. The effects of two levels of H2S and three levels of CO2 on the FCGR and fracture toughness were investigated. Frequency scan was conducted to obtain the frequency at which the FCGR would plateau. The highest frequency at which the FCGR attained a plateau will be used to determine the da/dN vs. DK curve. The data will be also be modeled based on the superposition principle of environmentally assisted fatigue to determine the role of hydrogen diffusion through the crack tip process zone versus surface reaction kinetics at the crack tip in generating hydrogen.