The phase field method is applied to study the microstructure evolution of Ni4Ti3 precipitates during stress-free and stress-assisted aging of bi-crystalline NiTi shape memory alloys (SAMs) with two different initial Ni-contents of 51.5at.% and 52.5at.% respectively.The simulation results indicate that during stress-free aging of the NiTi alloy with a low supersaturation of Ni (i.e., Ti-51.5at.%Ni), the Ni4Ti3 precipitates exhibit a heterogeneous distribution with a high number density of particles at the grain boundary, leaving most part the grain interior precipitates free, while for the NiTi alloy with a high supersaturation of Ni (i.e., Ti-52.5at.%Ni) the Ni4Ti3 precipitates show a homogeneous distribution across the entire simulation system.The stress-assisted aging can give rise to homogeneous distribution of the precipitates, regardless of initial Ni-content, however, the distribution of the variant type within the two grains is heterogeneous.