Abstract: High-temperature diffusion annealing treatment at different temperatures （1 120, 1 170, 1 220, 1 270 ℃） for different holding times （5, 10 h） was performed on 4Cr5Mo2V hot work die steel produced by electric arc smelting and electroslag remelting. The effect of annealing temperature and holding time on the dissolution behavior of primary carbides and grain size in 4Cr5Mo2V hot work die steel was studied. The results show that long strip-shaped, fishbone-like, nearly spherical and irregular blocky Mo-rich and V-rich primary carbides existing in the 4Cr5Mo2V steel could be eliminated by high-temperature diffusion annealing effectively. After annealing at 1 220 ℃ for 10 h, the Mo-rich primary carbides were completely dissolved, while the V-rich primary carbides could be basically dissolved at 1 270 ℃ for 10 h. This difference stemmed from the strong bonding energy between V and C, as well as the high melting point and thermal stability of V-rich carbides. A mathematical model for grain growth during high-temperature diffusion annealing of 4Cr5Mo2V steel was established. The model showed that the grain size increased exponentially with high-temperature diffusion annealing temperature and followed a power-law growth with holding time, and the temperature was the key parameter that dominated grain coarsening. Considering both the elimination effect of primary carbides and the avoidance of grain boundary overheating in the microstructure, the optimal high-temperature diffusion annealing process for 4Cr5Mo2V steel was 1 270 ℃ for 10 h.