There is an increasing demand for high strength ( more than 110ksi Gr.) tubing that are usable at high temperatures from 175 to 200oC for HPHT wells for mildly sour environments. Super 13Cr steels that are categorized as "13-5-2" in ISO13680 are typically used in HPHT wells containing CO2 with and without small amount of H2S. However the temperature limit of super 13Cr steels is equal to or less than 175oC. In this study the effect of alloying elements of stainless steels on general corrosion resistance at high temperature and environmentally assisted cracking resistance at high temperature and room temperature was investigated. It was found that Cr Ni and Cu are effective elements for improving resistance to general corrosion and cracking at high temperature in CO2 environment with and without small amount of H2S and Mo has beneficial effect on cracking resistance at room temperature in CO2 environment with small amount of H2S. From the results the optimization of chemical composition which can be suitable for HPHT usage in mildly sour environments will be discussed.