Abstract: Equilibrium phases in modified HP40 alloys and the effect of the alloying elements Nb, Ti and W in modified HP40 alloy on carbides precipitation behavior were investigated employing the Thermo-Calc software and Fe-alloys database. Chemical composition of the carbides was also calculated. The calculation results provide the theoretical bases for developing modified HP40 alloys with high structure stability. The results showed that Nb and Ti were the main forming elements of MC. With the increase of Nb and Ti, the precipitation content of MC was raised, the precipitation content of M23C6 and M7C3 were reduced, initial precipitation temperature of MC and M23C6 were raised. W elements were mainly distributed in the γ matrix at high temperature and would continue to segregate from γ matrix to M23C6 after long-term aging at the service temperature. With increasing the content of W, the precipitation content of M7C3 was reduced, initial precipitation temperature of M23C6 and transformation temperature from M7C3 to M23C6 were raised markedly.