Effects of isothermal aging on the microstructure and tensile behavior of Sn-3.0Ag-0.5Cu-0.2Co (SAC305-0.2Co) solder was explored, and compared with the standard Sn-3.0Ag-0.5Cu (SAC305) solder. The addition of Co resulted in the formation of CoSn2 phase in SAC305-0.2Co solder matrix. The isothermal aging treatment induced microstructural coarsening for both solders. With aging, the eutectic network has been translated into block or granular shape particles. Meanwhile, the outline of the primary Sn phase became less distinct. The tensile test results indicated, under as cast condition, the Co had little effect on the ultimate tensile strength (UTS) of SAC305-0.2Co solder, whereas it suppressed the ductility. After aging, the UTS of SAC305 was obviously decreased, whereas the elongation hardly changed. For the SAC305-0.2Co, the tensile behavior showed two trends: one was the decrease of the UTS, which was similar to that of SAC305; the other was the notable increase of elongation. The examination of the fracture surfaces indicated that isothermal aging could alter the fracture pattern of SAC305-0.2Co solder from brittle to ductile.