The goal of this research is to evaluate the effects of La addition on the microstructure and microhardness of Sn-Ag based solders. Sn-3.5Ag-xLa ternary alloy solders were prepared by adding 0-1.0wt% La into a Sn-3.5Ag alloy. Copper substrates were then dipped into these solders and aged at 150°C for up to 625 hours. The microstructure and microhardness of the as solidified solder and the aged solder/copper couples were investigated. Experimental results show that Sn-3.5Ag-xLa solders are composed of β-Sn, Ag3Sn, and LaSn 3 phases, and that their microstructure is refined with La addition. After isothermal storage, the Ag3Sn and IMC layer can be effectively depressed by adding a small amount of La element, and the size and amount of LaSn3 compounds does not change perceptibly with storage time. For as-cast solder, the addition of La increased the microhardness of the Sn-Ag solder due to the refining effect of Ag3Sn particles in the eutectic zone and increased formation of LaSn3 compound. After isothermal storage, the microhardness of solders was decreased with the increasing coarsening of Ag3Sn compounds as aging time was increased. However, coarsening of Ag3Sn compounds was retarded by La addition. Therefore, La addition helped to improve the microhardness and thermal resistance of the solder joints.