The　microstructures　and　mechanical　properties　of　SnAgCu+xCo　(x=0,　0.1,　0.2,　0.45　and　1.0)　composite　solder　joint　were　investigated.　The　effects　of　Co　element　on　the　undercooling　degrees　and　solidifying　phases　of　solder　joints　were　researched　by　Differential　Scanning　Calorimetry　(DSC).　Experimental　results　indicate　that　the　thicknesses　of　intermetallic　compound　layers　in　SnAgCu+xCo　(x=0.1,　0.2)　composite　solders　were　thinner　than　that　of　the　Sn-3.0Ag-0.5Cu　solder.　After　150　degrees　C　aging　for　500　h,　the　shear　strengths　of　all　SnAgCu+xCo　composite　solder　joints　were　higher　than　that　of　the　Sn-3.0Ag-0.5Cu　solder　joint.　It　suggests　that　the　Co　element　could　improve　the　service　reliability　of　SnAgCu　solder　joint.　Besides,　Co　could　increase　solder＇s　solidifying　point　and　decrease　undercooling　degrees　effectively.　In　a　word,　the　mechanical　properties　and　service　reliabilities　of　Sn-3.0Ag-0.5Cu-0.2Co　composite　solder　joint　is　optimal.