Effect of the in situ post weld heat treatment (PWHT) was investigated on the flash profile, austenite/ferrite phase balance, and mechanical properties of the upset resistance dissimilar weld between Fe-Cr-Ni and Fe-Cr stainless steels rods. In order to explore the effect of the heat treatment on the joint strength, two as-welded samples with low strength (116 MPa) and high strength (372 MPa) were used. The results showed that in situ PWHT was beneficial for both welded samples, though in different ways. For the weld with low strength, PWHT improved the joint strength (∼130% increase in the optimum condition compared with the as-welded sample) due to the increase in the size of the flash and the related bonded area at the joint interface. However, ferrite percent in the weld zone increased from ∼50% up to ∼70%. For the sample with the high strength, ferrite/austenite phase balance was restored at an optimum condition of PWHT. However, the joint strength decreased slightly (less than 5%) due to the grain growth in the Fe-Cr rod, i.e., the fracture location. Fracture analysis was used for justification of the variations in the joint strength. For both Fe-Cr-Ni side and Fe-Cr side of the welds, in situ PWHT generally reduced the hardness. This observation is discussed in light of the simultaneous effects of the grain growth and formation of little martensite.