The materials used were Ti-50.0, 50.5 and 51.0at%Ni alloys which were cold rolled and aged at 400°C for 1 hr. Neutron irradiation was performed in the Japan Materials Test Reactor (JMTR) up to a dose of 1.2 × 1024n/m2 at cooling water temperature (about 60°C), which meant the specimens were irradiated in the parent phase (B2). After the irradiation, micro-Vickers hardness measurements, microstructure observations by transmission electron microscope (TEM) and positron annihilation spectrometry (PAS) measurements were carried out. Differential scanning calorimetry (DSC) measurements were taken at temperatures between -120°C and 120°C. After the irradiation, all the alloys show extremely high irradiation hardening: the estimated increase in the yield stress of Ti-50Ni alloy is almost 1GPa. Although the hardness of the Ti-Ni alloys depends on the alloy composition in the unirradiated condition, all the alloys reach the same hardness value after the irradiation. DSC measurements revealed that the martensitic transformation was completely suppressed by the irradiation in all the alloys. PAS study revealed that no structural vacancies existed in the alloys before the irradiation, and that vacancies were formed after the irradiation. Microstructural observations indicate that disordered regions, which were considered to be amorphous, were homogeneously distributed in the irradiated parent ordered phase, accompanied by an appearance of the halo ring on the diffraction pattern. The irradiation effects disappeared following a post-irradiation anneal at 250°C for 1 hr. The recovery of the martensitic transformation by post-irradiation annealing is attributed to the migration of vacancies that causes the reordering. It is expected that Ti-Ni alloys are potential self-restorative materials for the irradiation above 250°C where vacancies are mobile.