The cooling rate has a strong influence on the volume fraction, size and distribution of primary α phase of TA15 titanium alloy, which play a decisive role in the properties of the material. Thus, the purpose of this paper is to quantitatively investigate the effect rules of the cooling rate on the primary α phase of TA15 titanium alloy by the α/β heat treatment experiment and the mathematical modeling based on a diffusion model. The obtained results show that: (1) the final volume fraction and size of the primary α phase both increase due to more growth time of the primary α phase resulting from the decrease of the cooling rate; (2) the growth rate of primary α phase increases with cooling rate because of the larger supersaturation under higher cooling rate, and the growth rate increases first and then decreases at a certain cooling rate; and (3) the final size distribution of the primary α particles becomes more uniform with the decrease of cooling rate. ► The volume fraction of primary α phase of TA15 alloy is sensitive to cooling rate. ► The growth rate of primary α phase increases with the increase of cooling rate. ► Under similar conditions, TA15 alloy produces less primary α phase than Ti–6Al–4V. ► The particle size distribution becomes more uniform with decreasing cooling rate.