Two different heats of modified 9Cr-1Mo steel, one without boron (P91) and the other with controlled addition of boron (P91B) and very low nitrogen were used for the present study. Microstructure of P91 steel, annealed at 875°C (inter-critical region) consisted of very fine prior austenite grains without lath martensite. Grain size increased with increase in heat treatment temperature and showed coarse prior austenite grains with clearly defined lath after 900°C heat treatment. In contrast, the microstructures of P91B steel subjected to identical heat treatment did not vary much with heat treatment temperature, prior austenite grain size remaining similar to that observed in the as received material. Significant improvement in the creep properties of P91B steel compared to that of P91 steel subjected to inter-critical temperature is explained based on the difference in microstructures observed in these two steels after the heat treatment. © 2013 The Authors.