Dissimilar metal welds (DMWs) are commonly used in the pressure vessel nozzle to safe-end weld between the ferritic low-alloy steels (LAS) and the austenitic stainless steels (SS), using a nickel-base filler metal. The complex DMW interface consists of different microstructural regions including, for instance, the heat-affected zone (HAZ), carbon-depleted zone (CDZ), carbon build-up at fusion boundary, partially melted zone (PMZ) and carbide precipitation zone. There is still knowledge lacking on the microstructural characteristics of the interface of DMWs upon post-weld heat treatment (PWHT). DMWs are potential concerns regarding the structural integrity of the nuclear power systems. In particular, the LAS/nickel-base alloy weld metal interface is known to develop a local strength mismatch upon PWHT and during long-term ageing. A significant chemical composition gradient, especially in terms of carbon (C) and chromium (Cr) associated with a complex microstructure have been observed to form at the interfaces. The different welding orientation, heat transfer and PWHT can result in different microstructure and mechanical properties.