The dependence of microstructural evolution on solute atom concentration in Ni alloys was investigated by positron annihilation lifetime measurements. The positron annihilation lifetimes in pure Ni, Ni−0.05at.%Si, Ni−0.05at.%Sn, Ni−Cu, and Ni−Ge alloys were about 400ps even at a low irradiation dose of 3×10 −4 dpa, indicating the presence of microvoids in these alloys. The size of vacancy clusters in Ni−Si and Ni−Sn alloys decreased with an increase in the solute atom concentration at irradiation doses less than 0.1dpa; vacancy clusters started to grow at an irradiation dose of about 0.1dpa. In Ni−2at.%Si, irradiation-induced segregation was detected by positron annihilation coincidence Doppler broadening measurements. This segregation suppressed one-dimensional (1-D) motion of the interstitial clusters and promoted mutual annihilation of point defects. The frequency and mean free path of the 1-D motion depended on the solute atom concentration and the amount of segregation.