Solute atoms greatly influence irradiation damage structures. For example, voids are formed in pure Ni at 573 K, but they are not formed in Ni-Sn alloys irradiated by fission neutrons at 573 K to 0.2 dpa. In this study, the interaction between solute atoms and vacancies and correlation between void growth and solute atoms in neutron irradiated Ni-Sn alloys were investigated by positron annihilation spectroscopy. Positron annihilation lifetime measurements showed that void density was lower in Ni-0.05 at%Sn than pure Ni. In Ni-0.3 at%Sn, single vacancies, di-vacancies and small stacking fault tetrahedra (SFTs) were formed and the size did not change below 0.1 dpa. In Ni-2 at%Sn, single vacancies and SFTs were formed and the size also did not change below 0.1 dpa. In Ni-Sn alloys, the total intensities of lifetime related to vacancy type defects were more than 60%. Vacancy concentration was higher than in pure Ni and a large number of positrons annihilated at them. From positron annihilation coincidence Doppler broadening (CDB) measurements, the ratio of annihilation of positrons and low momentum electrons increased in Ni-Sn alloys compared with pure Ni. The ratio increased with increasing the solute atom concentration. As positron affinity of Sn is lower than that of Ni, positrons are attracted to Sn atoms more strongly than Ni. Complexes of vacancy clusters and Sn atoms are formed and the number of Sn atoms adjacent to vacancy clusters increases with the solute atom concentration. It is expected that Sn atoms change the electron state of vacancy clusters, which leads to the increase of annihilation ratio between positrons and low momentum electrons.