A investigation was done to determine the effect of post-weld heat treatment on the microstructure and mechanical properties of ERNiCrMo-3 weld metals produced by metal inert gas shielded welding. The microstructure investigation revealed that the elements were irregularly distributed and severely segregated between the interdendritic region and dendrite core. Along the interdendritic region, a sizable number of secondary phases are also observed. The dissolution of Laves phase during the post-weld heat treatment would promote the precipitation of the M 23 C 6 carbide along the grain boundaries. The post-weld heat treatment seemly deteriorated the tensile properties of weld metal and changed the fracture mode from transgranular to a mixed mode of transgranular and intergranular fracture. The M 23 C 6 carbide precipitation at the grain boundaries during the aging heat treatment may be blamed for the decline in mechanical properties.