A proposal of marine engine indicated power uncertainty evaluation method was presented. The uncertainties of the indicated power definitional formula components were evaluated. Methods of uncertainty estimation for measurement carried out in industrial conditions were proposed. A new component called a phase shift angle, in modified formula for the piston kinetic was introduced. The indicated power uncertainty was divided into two calculable parts called uncertainty category I and uncertainty category II. The results of the uncertainties determined for two marine engines: trunk piston and cross-head type were presented. The influence of the individual component of indicated power formula for the overall indicated power uncertainty value was shown.