To clarify the effect of Sn substitution for Ni of LaNi5 on the lattice defect formation during the hydrogenation and dehydrogenation processes, in-situ positron lifetime measurements were per-formed in LaNi4.93Sn0.27. During the hydrogenation, the mean positron lifetime, τm, monotonically increased up to 175 ps which is almost same as calculated positron lifetime for vacancy. This shows vacancy introduction by hydrogenation. The τm mean positron lifetime decreased down to 135 ps with hydrogen content during the dehydrogenation. It shows that the vacancies are removed from the lattice during the dehydrogenation. These results show that vacancies in LaNi4.93Sn0.27 are intro-duced and removed reversibly during the hydrogenation and dehydrogenation. The concentra-tions of vacancy and dislocation were 1 × 10−5 and 6 × 109 cm−2, respectively. These values are two orders lower than those in LaNi5.