Type IV cracking in heat affected zone (HAZ) of weldment is a problem for advanced high Cr ferritic steels. It is important to predict initiation and growth of creep voids and cracks in weldment. Present paper clarified the Type IV crack initiation and growth properties in fine-grained HAZ of weldments for tungsten strengthened high Cr steel. On the basis of experimental results, the programs that simulate type IV fracture have been developed. The effect of HAZ width on vacancy diffusion and crack initiation was discussed.