The dissimilar welds joint of 2.25Cr-1Mo steel and lean duplex LDX2101 steel was prepared using the Gas Tungsten Arc Welding (GTAW) process and employing the Inconel 617 filler. The microstructural stability of the dissimilar welds joint (DWJ) was studied using the optical microscope and field emission scanning electron microscope (FESEM) for as-welded (AW) and post-weld heat treatment (PWHT) condition. The PWHT was employed at 760 °C (PW 1) and 810 °C (PW 2) for 2 h of duration. The application of the heat-treatment resulted in the formation of the soft zone at the interface of weld metal and HAZ in 2.25 Cr-1Mo side, i.e., low Cr side. The mechanical behavior of the DWJs was also studied for AW and PWHT condition. A measurable change in the width and hardness of the soft and hard zones was measured with an increase in PWHT temperature. For weld metal, the minimum average hardness was measured 237 HV in AW and a maximum of 248 HV for PW 2. In the AW condition, the average CIT of the weld metal was measured 46.5 ± 3.5 J, which meets the minimum recommended value as per EN 1557:1999 standards. The PWHT showed a drastic reduction in CIT and a minimum of 34 ± 3 J was measured for PW 2. The CIT of the 2.25Cr-1Mo HAZ (notch adjacent to fusion boundary includes the soft ferrite region) was measured 45 ± 5 J in AW and get increased drastically after the PWHT and maximum was measured for PW 2 (86 ± 5 J). © 2020 Elsevier B.V.