0.7 mm thickness invar alloy sheet is welded at 2.2 m/min laser scanning speed by using butt and lap joint. Microstructure analysis is conducted by optical microscope and scanning electric microscope. Hot tearing mechanism and susceptibility of butt and lap joint are discussed, respectively. Results show that the weld metal is composed of as-cast single phase dendrite austenite. Some hot tearing is in the weld center of butt joint, but none is in the lap joint. At the last solidifying stage of impinging dendrite, generally, dendrite grain boundary energy γgb is always two times larger than liquid/solid interfacial energy γsl of liquid film. The liquid film coalescence will not occur until adequate undercooling ΔTb, which results in hot tearing of butt joint under welding residual tensile stress. However, the remained liquid of upper weld pool can flow down and fill into torn liquid film, then lap joint has a lower hot tearing susceptibility.