Characterizations of the microstructure and mechanical property of the fusion boundary region of an Alloy 182-A533B low alloy steel (LAS) dissimilar weld joint were conducted. The existence of type-II boundary that parallels to the fusion boundary in the dilution zone (DZ) of Alloy 182 within a distance of about 50 μm was observed. The chemical composition transition was found in the narrow zone between the type-II and the fusion boundaries. Highest hardness was also found in this narrow zone in the fusion boundary region, implying a high residual strain level in the narrow zone. Lath martensite was observed in both sides of the type-II boundary in DZ, suggesting the localized enrichment of Fe and C adjacent to the type-II boundary due to the diffusion from LAS to DZ. Massive precipitations of carbides were observed on the type-II boundary but not on the fusion boundary. In addition, the orientation relationship at the fusion boundary between the lath martensite in the heat-affected zone in LAS and the austenite in the DZ was found to be Bain, K-S and N-W relationships.