Abstract: Spontaneous growth of ZnO whiskers in Sn-9Zn/Cu lead-free solder joints was investigated. The results indicate that due to the oxidation of rich-Zn phases at room temperature, ZnO whisker will spontaneously grow on the surface of Zn-rich phase in Sn-9Zn/Cu lead-free solder joints, and these whiskers showed different morphologies like needle-type, bulk-type and flower-type, etc. Based on FIB test of ZnO whisker, it was found that voids appeared in the Zn-rich phase, which was induced by the diffusion of Zn atoms. In addition, during reaction between Zn and O, volume shrinkage of Zn-rich phase might happen, and the tensile stress could be generated in the structure which was the driving force of ZnO whisker growth.