Abstract: The effect of nano Ag particles on the wettability, microstructure, interface IMC and mechanical properties of Sn-58Bi soldered joints was studied. The results showed that the addition of Ag particles can refine the microstructure of the soldered joints. The microstructure of the composite solder shows the trend of the first refinement after coarsening with the increase of Ag content. The addition of Ag particles increases the thickness of the interface IMC, and the IMC thickness increases with the increase of the Ag particle content in the composite solder. Ag particles addition can improve the spreadability and it shows a trend of first increase and then decrease with the content of Ag particles being increased. The tensile properties of soldered joints can be improved by adding proper amount of Ag particles, and the tensile properties of the soldered joints increase first and then decrease with the increase of the content of Ag particles. The optimum addition amount of Ag for Sn-58Bi solder is about 0.5wt.%.