The size and morphology of Ti₃Ni₄ precipitates, as well as their transformation behavior after isothermal aging, were studied in a Ti-50.7 аt.%Ni shape memory alloy with various B2 austenite grain sizes. A 0.3 mm thick band with an accumulated strain of ε = 44% (e = 0.6) obtained by cold rolling was used. The samples were then solution-treated at 600–800°С for 0.3–1 h with subsequent quenching in water to obtain the structure with different grain size (GS) from 5 to 15 μm. The samples were then subjected to isothermal annealing at 430 °C for 1, 3 and 10 h. The SEM observations were carried out using a JSM-6460LV. The microstructure was studied using a JEOL 2100 TEM. Characteristic temperatures were measured using a “Mettler Toledo” DSC. The results of the microstructure study prove that the GS strongly affects Ti₃Ni₄ particle size and morphology, as well as the degree of microscale microstructure heterogeneity, transformation kinetics and transformation sequences. This influence is non-unique and depends on the duration of isothermal annealing. After 1 h of aging, the particle thickness and diameter maintain the nanometer range in the whole GS range of 5–15 μm. The increase of aging time to 3 and 10 h leads to localization of nanosized precipitates in the grain-boundary region, with the width of these regions decreasing with increasing GS. After aging for 1 h, the samples with the GS of 5 μm show the B2→R transformation. The increase in the GS to 15 μm and aging duration to 10 h is accompanied by multiplication of martensitic transformations to four stages. The study explains the influence of GS on the precipitated particle size and morphology, transformation kinetics and staging.