For further improvement in thermal efficiency of a heavy-duty diesel engine, heat balance calculation based on the first law of thermodynamics was applied to various engine operating conditions to find out the measures to improve brake work of the engine. And also, comparison of the heat loss from the wall calculated by the heat balance analysis with the heat loss calculated by the empirical formulas, i.e. Woschni’s / Eichelberg’s heat transfer coefficient was then carried out to explain a key factor for the difference between them in various engine test conditions.