The effect of environment relative humidity (RH) on the corrosion behavior of Sn-0.7Cu solder was investigated by the weight loss method and surface characterization techniques. In this study, RH ranging from 50% to 98% was researched. The results show that the corrosion degree is getting worse from 50% RH to 90% RH. However, the corrosion rate at 98% RH is lower than that at 90% RH, which may be ascribed to the degree of oxygen diffusion in electrolyte layers of different thicknesses. The microstructure characterization proved that the corrosion products exhibit a superimposed growth and present the same corrosion degree variation trend as the corrosion kinetics. The compositions of corrosion products are examined, and Sn 21 Cl 16 (OH) 14 O 6 , SnO, and SnO 2 are the main corrosion products.