The hardness and microstructure evolution of a 8% Cr cold work tool steel during tempering for 40 h were investigated. Transmission electron microscope examinations showed that M_3C carbides precipitated from supersaturated martensite after tempering at 350 ℃. When the tempering temperature was higher than 520 ℃,the M_(23)C_6 carbides precipitated to substitute for M_3C carbides. After ageing at the temperature of 520 ℃ for 40 h,it was observed that very fine and dense secondary Mo_2C precipitates were precipitated. Thus,it can be concluded that the early stage of Mo_2C-carbide precipitation is like to be Gunier-Preston( G-P) zone formed by [Mo-C] segregation group which is responsible for the secondary hardening peak at 520 ℃. Overageing at 700 ℃ resulted in recovery of martensitic microstructure and precipitation of M_(23)C_6 carbides.When ageing at 700 ℃ for more than 20 h,recrystallization occurred,which resulted in a change of the matrix morphology from martensitic plates to equiaxed ferrite. It was noticed that the size of recrystallized grain / subgrain was very fine,which was attributed to the pinning effect of M_(23)C_6 precipitates. The hardness and microstructure evolution of a 8% Cr cold work tool steel during tempering for 40 h were investigated. Transmission electron microscope examinations showed that M_3C carbides precipitated from supersaturated martensite after tempering at 350 ° C. When the tempering temperature was higher than 520 ° C., The M_ (23) C_6 carbides precipitated to substitute for M_3C carbides. After aging at the temperature of 520 ° C for 40 h, it was observed that very fine and dense secondary Mo_2C precipitates were precipitated. Thus, it can be concluded that the early stage of Mo_2C-carbide precipitation is like to be Gunier-Preston (GP) zone formed by [Mo-C] segregation group which is responsible for the secondary hardening peak at 520 ° C Overageing at 700 ° C resulted in recovery of martensitic microstructure and precipitation of M_ (23) C_6 carbides. Growth at 700 ℃ for more than 20 h, recrystallization occurred, which resulted in a change of the matrix morphology from martensitic pla tes to equiaxed ferrite. It was noticed that the size of recrystallized grain / subgrain was very fine, which was attributed to the pinning effect of M_ (23) C_6 precipitates.