Present paper describes the microstructural and mechanical properties of creep strength enhanced Cr–Mo P91 steel subjected to a regime of tempering time 720–3000 h. Study on the effect of varying tempering time on microstructure revealed increase in size of precipitates, amount of precipitates and grain size. It was also observed that compared to particle size, effect of tempering time was more pronounced on the grain size. The various phases that formed in the tempered P91 steel were analyzed by X-ray diffraction. Yield strength and tensile strength were observed to be decreasing with increase in tempering time. Increase in duration of tempering time led to decrease in impact toughness of P91 specimens possibly due to cracking of secondary phase particles during straining and decohesion at precipitates and matrix interface. The tempering time also observed to be affecting the hardness of P91 steel but less pronounced compared to the yield strength and tensile strength. Tensile fracture and impact toughness fracture surfaces were studied by using scanning electron microscope. The fracture surface of tempered P91 tensile specimens indicated increased dimples and presence of fewer cleavage facets. The fracture surface of P91impact toughness specimens exhibited both inter lath-decohesion and ductile dimple tearing. © 2016, The Indian Institute of Metals - IIM.