The effects of the post weld heat treatment (PWHT) on the microstructure and hardness of the underwater wet welding (UWW) SS400 steel have been evaluated. The UWW was performed using Shielded Metal Arc Welding (SMAW) in water depth of 5.0 m, while PWHT was carried out on underwater welded specimen with temperature of 660 oC for 75 minutes. The result showed that the PWHT caused increasing the grain size of the microstructure and decreasing in hardness.