Abstract Microstructure evolution was studied in a 3%Co modified P911 heat resistant steel during creep tests at 873 and 923K to failure, which occurred in 4103 and 4743h, respectively. The tempered martensite lath structure consisted of packets, blocks and laths. The average spacing of high-angle boundaries and the mean transverse lath size were about 1.9μm and 360nm, respectively. Various second phase particles precipitated upon tempering. Fine M(C, N) carbonitrides with an average size of about 30nm were homogeneously distributed throughout the tempered martensite laths, while relatively coarse M23C6 carbide particles (average size 120nm) were located at internal boundaries. The tempered martensite lath structure was rather stable upon aging for about 4×103h. The boundary precipitates of M23C6 and Laves phases, which appeared during creep tests, exerted a high pinning pressure on low-angle lath boundaries and high-angle packet/block boundaries. The growth of martensite structural elements during the tests correlated with the coarsening of second phase particles. Quantitative relations of pinning and driving pressures for low- and high-angle boundaries are discussed.