Recently, magnesium alloy is in the limelight as ECological material with high ability of recycling and lightweight property. Especially, because of outstanding properties as a structural material, the magnesium alloy is in great demand. Under these circumstances, the technical researches and developments of welding process for magnesium alloy welding have great significance to expand industrial application of magnesium. In magnesium alloy welding, arc welding process is generally used. However, heat input by the welding arc affects the magnesium alloy plates, and as the result it makes wide heat affected zone and large distortion of the welded joint. As one of the possible means to improve quality of the welded joint, application of laser welding process may be recommended. However, the low boiling point of magnesium generates some weld defects, such as porosity and solidification cracking. Furthermore, the small laser beam diameter is very sensitive to edge preparation in butt joint. Accordingly, application of laser/ arc hybrid welding process to magnesium alloy welding was proposed. The laser/ arc hybrid welding process is a new process combining the laser beam and the arc as welding heat source. The laser beam and arc influence and assist one another. By application of hybrid welding, synergistic effects are achievable, and disadvantage of the respective processes can be compensated. In this study, welding of AZ31B magnesium alloy thin plates using YAG laser/ TIG arc hybrid welding process was investigated. In order to confirm the properties of the welded joints, tensile testing was performed.