Abstract: Behaviors of Sn whisker growth in Sn-Zn-Ga-Pr lead-free solder under different oxidation conditions for the same aging period (30 days) were investigated by environmental testing (room ambiance,temperature humidity and oxidation-free tests) and microstructure evaluation. It was found that the whiskers grew in hillock-type morphology under temperature humidity and in nodule-type under N₂ atmosphere while the typical needle-like whisker grew in room ambience. It was discussed that the difference in whisker growth under different aging conditions is attributed to the different oxidation of rare earth (RE)-rich phase. The RE-rich PrSn₃ phase experienced rapid oxidation and corrosion under temperature humidity,which led to higher driving force and more Sn sources to ensure hillock growth than that under ambient condition while the slow oxidation of PrSn₃ led to lower driving force and less Sn atoms under N₂ atmosphere.