The method of calculation of the quantitative content of disperse inclusions of strengthening phases with use electron microscopy in heat resisting alloys of 25Cr-35Ni-C-Si-Nb system is considered. The method with advantage is tested for an alloy 0.45C-26Cr-33Ni-2Si-2Nb. Experimental curve contents of chromium and niobium carbides in alloy structure after high-temperature endurance of various duration at 1150 °C are submitted.