In powders with different contents (0.5-10 wt.%) were melted into the Sn-3.0Ag-0.5Cu (SAC305) solder to change the microstructure and thus improve the properties of the solder. The results showed that β-Sn(In), Ag₃(Sn,In) and Cu₆(Sn,In)₅ phases existed in all the In-added solders, and an additional phase of InSn₄ was found in the SAC305-10In solder. With increasing In addition amount, the β-Sn nucleation sites were increased and the nucleation model was changed from {101} to {301}, which refined the β-Sn grains and transformed the β-Sn morphology from large grains to interlaced grains and finally to multiple grains. As a result, a combined effect of fine grain strengthening and solid solution strengthening was achieved to increase the microhardness of the SAC305-xIn solders with increasing In addition amount.