Effects of environmental factors on general corrosion property of API and modified 13Cr steels were systematically investigated. An essential reaction determining the corrosion rate of steels in CO2 environment is considered to be a direct reduction reaction of carbonic acid. In the absence of protective film of corrosion product or passivity,the corrosion rate of steels can be written by an Arrhenius–type equation including parameters of activity of carbonic acid and temperature,and using this equation a safety use limit of each steel in various oil and gas environments can be estimated. Keywords : General corrosion, pH, H2S, CO2,HCO3 H2C03 ,Cl ,temperature, Martensitic stainless steel,13Cr steel, Modified 13Cr steel