During the engine experimental investigation usually was obtained engine effective power and rotational speed. In order to obtain engine mechanical efficiency and engine indicated power from experimental data, must be used equation for calculation of engine mean pressure of mechanical losses along with its proper coefficients. This equation gives correct values if engine mechanical efficiency and engine indicated power was obtained in the range ±1.5%, when compared with experimentally obtained ones. For marine slow speed twostroke diesel engine 6S50MC MAN B&W was analyzed this equation along with its coefficients during the whole range of engine loads. The mean pressure of mechanical losses equation, when obtained coefficients was applied, gives a percentage difference in the range ±0.79% for the engine mechanical efficiency and percentage difference in the range ±0.85% for the engine indicated power when compared with experimentally obtained values. For the higher engine loads presented equation is even more accurate and precise.