The evolution of microstructure and its effects on the mechanical properties of modified 9%Cr-1%Mo steel during heat-treating at 1050 degrees C for 15 min and then isothermal heat treatment at 380 similar to 760 degrees C with subsequent air-cooling have been investigated. For the microstructural. and mechanical property analyses, OM, SEM, EDS, XRD, hardness and impact tests were used. in accordance with the severity of the heat-treatment, the microstructure evolved from the untransformed martensite to the partially transformed dual phases of martensite and ferrite, and then fully transformed to ferrite. Impact values at ambient temperature for specimens isothermally heat-treated at 320 - 380 degrees C, predominantly at about 350 degrees C were lower than others' with similar martensitic structure. The partially transformed specimens with dual phases of martensite and ferrite also showed lower impact values than samples with untransformed with martensitic, and transformed with ferritic structures.