The chemical and electrochemical equilibrium of oxygen-chlorine-titanium system in the presence of gaseous phase were investigated. Many species, which consisted of oxygen, chlorine and titanium, were considered. Various thermodynamic equilibriums were calculated in the different pressures and temperatures. Calculation results were shown as E-T diagram. This diagram will be used as important tools for corrosion study and titanium production, and it is also used to thermodynamically determine the existence areas of various species and so on.