The present research work deals with the results of influence of different heat treatment conditions on the microstructure stability and mechanical properties of high-chromium X10CrMoVNNB9-1(P91) cast and forged (C&F) P91 steel. The C&F P91 steel was subjected to various tempering condition in the temperature range of 350–760 °C and tempering time of 2 h, after the normalizing at 1040 °C for 40 min. The steel was also subjected to furnace cooled and water quenched heat treatment. Tempering at 650 and 760 °C produced the fully tempered lath martensitic structure with M23C6, M7C3, M3C and MX precipitates along the prior austenite grain boundaries, lath boundaries and matrix region. Tempering at 350 and 1000 °C produced the partially tempered columnar laths and untempered columnar laths martensite, respectively. The tempering time was also varied from 2 to 8 h for fixed tempering temperature of 760 °C. The optimum microstructure evolution was obtained for 6 h of tempering at 760 °C that led to improved mechanical properties. © 2017 The Indian Institute of Metals - IIM