Abstract: AZ31B magnesium alloy and 6061 aluminum alloy were welded by cold metal transfer(CMT) welding method with pure copper(HS201) as the filler metal. Shear strength and micro-hardness of the lap joint were tested. The microstructure and fracture morphology of joint were studied by means of scanning electron microscope(SEM) and energy dispersive Xray(EDX). The results showed that good weld with the highest shear bonding strength of 27.9MPa was obtained under the welding current of 109 A, welding voltage of 10.9 V, wire feed speed of 4.9 mm/s, welding speed of 0.45 m/min. Intermetallic compounds of Mg₂Cu and MgCu₂ were formed in the Mg-Cu side where was oxidized. Al-Cu-Mg ternary intermetallic compounds and Al₂Cu were formed on the Al-Cu side. Micro-hardness in both sides of fusion zone was suddenly increased due to intermetallic compounds. Maximum micro-hardness of Al-Cu side and Mg-Cu side were 242 HV and 347 HV respectively. The fracture occurred in the fusion zone of Mg side, and fracture nature was brittle fracture. Oxidation in the welding process and plenty of intermetallic compounds distributed continuously in the fusion zone, and resulted in the fracture.