Abstract: The measurements of the S-shaped parameter of Doppler-broadening of positron annihilation were carried out for the near-equiatomic Ti-Ni alloys of different concentrations subjected to quenching at different temperatures and then annealed at different temperatures after quenching at 1000℃. The. result showed:(1) the change of S-parameter in the high temperature range of the quenching curves and in the low temperature range of the annealing curves could be explained by the mechanism of single vacancy, (2) for the Ti-Ni alloys, the positron could detect the precipitation in these probable ranges of temperature:300°~700℃ in Ti-49.0 at% Ni,350°~700℃ in Ti-50.0 at%Ni, and 400°~700℃ in Ti-50.5at% Ni, Ti-51.0at% Ni and Ti-51.5 at% Ni, (3) the amount of "defects" on the Ni-rich side was changed more easily by the conditions of the treatment than that on the Ti-rich side, and (4) the amount of "defects" on the Ni-rich side was lower than that on the Ti-rich side under the same condition of heat-treatment.