This present work investigated the influence of welding conditions, such as welding current, diameter of welding wire on the microstructural change, and mechanical properties of TIG welded joint in extruded AZ31B Mg plate. It was found that a good and sound welded joint was achieved in all welding conditions. The grain size decreased with increasing welding current and decreasing diameter of welding wire. In addition, the second phases were homogeneously distributed in the grain and grain boundary as the welding current and the diameter of the welding wire were decreased. β discontinuous precipitates were observed in the welded joint; this microstructure has not been reported by previous researches investigating AZ31B Mg alloy. The hardness value was affected by the existence state of the second phase and the hardness of the welded joint region was lower than the other regions in welded AZ31B Mg alloy. The strength of the welded joint region was influenced by the grain size and had more than 90 %, compared to that of ASTM standard specification.