Transformations of strengthening carbide phases in the structure of a heat resisting heat-resistant alloy 25Cr-35Ni-C-Si-Nb of the HP40Nb series during long endurance at a temperature of 1150 °C were studied by the method of a quantitative metallography with use of raster electronic microscopy and the X-ray spectral microanalysis. Dependences of change of quantity and morphological parameters of carbide phases in structure of the alloy at high-temperature operation are received and explained. It is established that the greatest quantitative and morphological changes of a carbide phase happen during the first 2-10h enduranceofa cast alloy.Itis shown that the increasein the maintenanceof carbide phasesinthe structure of a heat resisting HP40Nballoyat temperaturesof operationis connectedwith consecutive transformation of the eutectic carbides of chrome in the direction: M7C3→M23C6→М6C. The carbon which is released as a result of transformation of carbides of chrome forms in matrix solid solution secondary disperse a compact form carbide inclusions.