The microdefects and free electron densities in B2, R and B19’ phases of Ni50.78Ti49.22 alloy have been studied by positron lifetime measurements. Comparing the lifetime parameters of the Ni50.78Ti49.22 alloy measured at 285 and 227 K, it is found that the free electron density of the R phase is lower than that of the B2 phase; the open volume of the defects for the R phase is larger, while the concentration of these defects is lower than that of the B2 phase. The Ni50.78Ti49.22 alloy is the B19’ phase at 100 K. In comparison with the R phase, the free electron density of the B19’ phase increases, the open volume of defects for the B19’ phase decreases, and the concentration of these defects increases. It is found that the microdefects and the free electron density play important roles during the multi-step transformations (B2→R→B19’) for the Ni50.78Ti49.22 alloy with the temperature decreasing.