Abstract: The isothermal transformation curve of undercooling austenite of 4Cr13 plastic mold steel was tested, and the isothermal phase transformation characteristics of the steel were studied. The microstructure and hardness after pretreatment by different solution (holding at 1 010-1 090℃ for 2, 3 h) and spheroidization annealing (austenitization at 905℃ for 2 h and isothermal spheroidization at 700-770℃ for 5 h and 8 h) were analyzed, and the optimal process was obtained. The results show that the at 725℃ the test steel had the shortest nucleation and incubation period. After solution at 1 010℃, there were a lot of spherical carbides in the structure. After soluted at 1 070-1 090℃, the carbides were basically dissolved, and the martensite lath bundles were significantly coarsened. The optimal solution process was 1 050℃ ×3 h, and the carbides was dispersely distributed with relatively low proportion. As the isothermal spheroidization temperature increased, the number of chain shaped carbides decreased first and then increased, and the distribution of spherical carbides was more uniform after holding for 8 h. After solution at 1 050℃ for 3 h and spheroidization annealing by 905℃×2 h+750℃×8 h, the spheroidization effect of the test steel was the best; the proportion of carbides with a diameter less than 0.5 μm was about 78.92%, and the hardness was the lowest of about 192 HBW.