Abstract: Plasma arc welding of 4J36 low-expansion alloy was conducted. The microstructure and mechanical properties of welded joint were analyzed by using optical microscope (OM), scanning electron microscope (SEM),energy spectrum analysis (EDS),X-ray diffraction (XRD),and Rockwell hardness tester. The results indicate that the weld appearance was excellent and the tensile strength of the joint reached 93.6% of the base metal under appropriate parameters. The microstructure of the weld consisted of austenite with some strengthening phase γ' (Ni₃(Al,Ti)). The microstructure of the base metal was fine equiaxed grain,while that around the fusion line had dendrites structure. In the HAZ,the grains near the weld pool grew up due to overheating,but those near the base metal were fine equiaxed grains. The tensile fracture revealed obvious ductile features, and the HAZ had the minimum hardness.