Magnesium alloys have high strength-to-weight ratio compared with other metals and excellent recyclability compared with plastics. Because of the increasingly stringent environment regulation, restricted natural resources and the demand for energy saving, the use of the "ultra-light" magnesium alloys is steadily gaining importance. This in turn has resulted in more demands on welding technology for solving various problems in joining magnesium components. Plates of 3-5 mm in thickness were extracted from an AZ91D ingot and then butt joints of the plates were produced using TIG welding method. The TIG arc was also used to deposit welding beads on some of the thin plates. No cracking was found in the butt joints. However, hot cracking was always observed to propagate from the heat affected zone under the welding bead into the weld metal. Metallographic and fractographic evidence was obtained to show that the hot cracking is “liquation cracking” in the partially melted heat affected zone under the high thermal stresses. In the butt joints, the weld metal has the finest grains, highest strength and best ductility, and the heat affected zone was found to be the "weakest link". HAZs were also found to be most susceptible to corrosion.