The effect of combustion process on the pressure exponent N in gas turbine combustor efficiency similitude study is investigated by numerical simulation. It is suggested that for hydrocarbon fuel, the pressure exponent N may be selected as 1.6〜2.0 when the combustion process is reaction-controlled. When the combustion process is diffusion controlled it may be selected as 1.0〜1.4. The result of this paper would be useful for the modeling test of gas turbine combustor.