The　effects　of　rare-earth　(RE)　element　additions　on　the　tensile　deformation　mechanism　of　the　Sn-3.8Ag-0.7Cu　solder　alloy　have　been　investigated.　The　results　show　that　adding　RE　elements　can　remarkably　improve　the　tensile　strength　and　elongation　of　the　Sn-3.8Ag-0.7Cu　alloy.　The　increase　in　the　mechanical　properties　are　attributed　to　the　constraints　of　microcrack　growth　and　grain　boundary　sliding　in　the　eutectic　phase　as　well　as　the　relaxation　of　stress　concentration　in　the　beta-Sn　phase　due　to　the　addition　of　the　RE　elements.　It　is　considered　that　the　RE　elements　strengthen　the　eutectic　phase　and　increase　the　deformation　resistance　of　this　alloy.