The effect of cobalt, tungsten, and boron on interfacial energy of precipitate/ ferritic matrix in the 9% Cr martensitic steels on the base of creep tests at 650 degrees C under different applied stresses ranging from 80 to 220 MPa was investigated. An interfacial energy of M23C6 carbides, the Laves phase particles, and MX carbonitrides was estimated by comparison of theoretical curves obtained by Prisma software for the model steels for the exposure time of 2 9 10 4 h with experimental data measured by TEM in the gage sections of crept specimens. Addition of 3 wt% Co to Co-free 9Cr2W steel led to about 1.7 times increase in the interfacial energy of M23C6 carbides and MX carbonitrides, whereas Co did not effect on the interfacial energy of the Laves phase. Increasing W from 1.5 to 3 wt% in the Co-containing steels led to increase in the interfacial energy of the Laves phase up to 0.78 J m(-2) under long- term exposure, whereas it did not effect on the interfacial energy of M23C6 carbides and MX carbonitrides. In the steel with increased B up to 0.012 wt% and decreased N to 0.007 wt%, a strong decrease in the interfacial energy of M23C6 carbides to 0.12 J m(-2) occurred. Change in the interfacial energy of the precipitates was analyzed in comparison with coarsening rate constant.