There are increasing needs for using 125 ksi grade material for production casing in deep water applications. There are also commonly observed that many High Pressure/High Temperature (HPHT) and XHPHT (extreme high pressure and high temperature) wells contain small amount of H2S because of natural geological souring and anticipated reservoir souring due to water injection. This requires that casing material not only provide high strength but also good sulfide stress cracking (SSC) resistance. A previous study found that a double quench/tempered 125 ksi grade casing material with a thin wall thickness low yield strength and hardness can provide good SSC resistance in mild sour conditions when PH2S (H2S partial pressure) is lower than 0.5 psi and pH is higher than 4.0.In the present work a systematic laboratory study was done to more accurately assess the effects of wall thickness yield strength and hardness on the SSC resistance of a proprietary 125 ksi grade casing material under various combinations of hydrogen sulfide partial pressure (PH2S) temperature (T) and pH. The NACE TM 0177-Method D/DCB (double cantilever beam) tests were performed to numerically rank the SSC resistances of the materials with different wall thickness yield strength and hardness. Different arm displacements were also employed to obtain KISSC-Limit in selected environment. Through this investigation the knockdown in 125 ksi casing material SSC resistance due to wall thickness yield strength and hardness is determined.Keywords: sulfide stress cracking double cantilever beam test yield strength wall thickness hardness 125 ksi grade casing material Authors: Weiji Huang1 Fang Cao2 Cecilie Haarseth1 and Wei Sun1 1ExxonMobil Development Company Houston TX2ExxonMobil Research and Engineering Company Clinton NJ