Aging treatment strongly affects mechanical properties and martensitic transformation behavior of Ni-rich Ti-Ni alloys. In this study, when a Ti-50.9at%Ni alloy was subjected to low temperature aging treatment, a two-stage R-phase transformation was first found. In order to clarify the properties of two-stage R-phase transformation, the mechanical properties and microstructures were investigated using tensile tests, DSC and XRD. The specimens were solution-treated at 1073K for 3.6ks followed by age-treating at 473K for 0.6kssimilar to10800ks. The two-stage R-phase trans formation temperatures increased with increasing aging time, while the difference between the two-stage R-phase transformation temperatures became smaller. It is identified by XRD measurement that two peaks measured by DSC correspond to the R phase transformations. According to TEM observation, fine and dense Ti3Ni4 precipitates (<20nm) were found in the specimens which showed the two-stage R-phase transformation.