In the present study, the Gas Tungsten Arc Weld (GTAW) joint of P91 steel was produced using the dissimilar IN617 grade filler wire. The welded joints have been subjected to different post-weld heat treatment (PWHT) to obtain the stable microstructure and mechanical properties for P91 heat-affected zone (HAZ). The PWHT was performed at 750 degrees C (less than Ac1) and 800 degrees C (close to Ac1). The welded joint was characterized for mechanical properties using tensile test, hardness test and Charpy impact toughness test. In order to understand the role of the dissimilar filler metal and PWHT on the Charpy impact toughness of the weld metal, Charpy impact toughness tests were conducted at room temperature. The Charpy impact toughness of the weld metal was measured 55 +/- 4 J for the as-welded condition. The PWHT has observed almost a negligible effect on Charpy impact toughness value. The tensile strength was measured 638 MPa for the as-welded joint and improved significantly after the PWHT. The role of the filler metal and PWHT on heterogeneity in microstructure and hardness produced along the HAZ of P91 steel were investigated in detail. The microstructure features of HAZ (grain size, particle size, fraction area of the precipitates and particle to particle distance) were measured experimentally using the ImageJ software as a possible contributor to the hardening mechanism. The HAZ (CGHAZ) devoided from the carbide precipitates showed the peak hardness of about 395 HV in as-welded condition and the minimum of 189 HV was measured for inter-critical HAZ (ICHAZ). The PWHT at 800 degrees C produced the homogenous microstructure and a negligible hardness variation along the HAZ. The precipitate free delta-ferrite region was identified in the CGHAZ region near to fusion line for as-welded condition and remain unaffected during the PWHT.