A heat exchanger is a device used to transfer heat between two or more fluids and it is an integral part of any oil and gas refinery. Tube-to-tube sheet welding is the most critical activity in fabricating this equipment [1]. There are many factors which needs to be considered to produce a highly reliable weld joint. These factors include the type of the joint, weld joint design, selection of suitable filler wire, and robust inspection plan deployed during fabrication [1]. The task of selecting a suitable filler wire becomes much more challenging when the heat exchanger consists of dissimilar tube to tube sheet weld joints. This paper discusses a case study on the filler metal selection methodology followed for the dissimilar tube-to-tube sheet welding of SB163 UNS N08935 (With 35% of Ni) tube to Alloy 400 clad tube sheet for one of the heat exchangers