When the power or specific fuel consumption is estimated in design process, thermodynamic consideration for the estimation is generally insufficient. Hence, a theory that can estimate these performances accurately is investigated in this paper.As a result of investigation, it is clear that the effect of pumping loss in wide-opene throttle valve operation has to be excluded from the mechanical loss which is measured in the motoring test. It also becomes clear that a new coefficient called pumping loss coefficient ηP has to be considered for the negative work for pumping.From the foregoing, theoretical formulas for estimating the net power Pe and net specific fuel consumption be. which are formed with various efficiencies and coefficients are as follows: It is verified that the estimation from these formulas agree well with the experimental test values using stoichiometric mixture ratio. By using these formulas, the mechanism for improvement of specific fuel consumption in the lean burn engine is also examined by analyzing the effect of air/fuel ratio on Pe and be.