Tin whisker growth on the surface of ErSn3 phase was investigated during air and vacuum aging of ErSn3, respectively. The results indicate that tin whisker growth occurs on the surface of the oxidized ErSn3 phase during air aging. Moreover, the growth rate is slow and the distribution is uneven during room temperature aging; on the contrary, the growth rate of tin whiskers is fast and the distribution is even during 150 oC aging; however, no tin whisker is found on the surface of ErSn3 phase during vacuum aging. The volume expansion which is caused by diffusion of oxygen atoms into ErSn3 lattice during oxidization in air produces compressive stress that provides the driving force for tin whisker growth; thus tin whisker growth happens during air aging and no whisker is found during vacuum aging