Dissimilar　metal　laser　welding　of　magnesium　alloy　AZ31B　and　aluminum　alloy　6061　was　conducted　on　both　lap　and　butt　joints.　Magnesium　aluminides(MgAls)　β-Mg2Al3　and　γ-Mg17Al12　were　identified　in　laser　welds　which　affected　weldability　and　joint　properties.　The　weld　microstructure　exhibited　three　distinct　regions　as　a　narrow　fusion　band,　an　MgAl　depleted　zone　and　a　dense　MgAl　dendrite　zone.　The　fusion　band　is　composed　of　α-Mg　and　γ-Mg17Al12　eutectic　and　nonepitaxially　grown　MgAl　column　dendrites.　MgAl　depleted　and　dense　dendrite　zones　of　lap　welds　consist　of　P-Mg2Al3　phase　and　Al　solid　solution　matrix,　while　Mg　solid　solution　is　an　additional　matrix　constituent　in　butt　welds.　Limited　tensile　strength　is　achieved　of　the　butt　welds.　Failure　occurred　in　the　interfacial　region　of　MgAl　depleted　zone　and　dense　dendrite　zone.　The　marginal　weld　strength　is　attributed　to　the　relatively　tougher　MgAl　depleted　zone　characteristic　of　quasi-cleavage.