The present work aims to investigate the microstructure and mechanical behavior of the dissimilar welded joint (DWJ) of martensitic A 335-grade P91 steel and high alloy ferritic austenitic A 182 F69 steel produced by the gas tungsten arc welding (GTAW) process. The microstructural studies showed the variety of the microstructure across the DWJ, which formed mainly due to the solidification and solid-state phase transformation as a result of the weld thermal cycle. The microstructural studies were performed in as-welded as well as post-weld heat treatment (PWHT) conditions. PWHT has been conducted to improve the mechanical properties of the P91 heat-affected zone (HAZ) at 760 °C (PW 1) and 810 °C (PW 2) for 2 h. For the mechanical properties of the DWJ, tensile tests, and microhardness tests were conducted. The fractured tensile specimen has been characterized using the scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The centre region of the weld metal showed the equiaxed dendrites while the interface near the boat fusion line, on both the P91 and F69 sides, showed the cellular and columnar dendrites. PWHT has observed a negligible effect on the microstructure and hardness of WFZ and F69 HAZ, while a significant change was observed for P91 HAZ. The tensile strength of the DWJ was measured maximum (735 MPa) for as-welded condition and reduced after the PWHT and for PW 2, it was measured lower than that of P91 and F69 base metal. The failure of the tensile specimen was also noticed in the soft inter-critical HAZ (ICHAZ) region for PW 2 condition. © 2020 Elsevier Ltd