Abstract: In order to study the growth kinetics of α phase precipitation in TC21 titanium alloy during ageing treatment, the relationship between the elemental composition of β and α phase and aging temperature was obtained using JMatPro software. The equation between the α phase growth rate and aging temperature of 500-900 ℃ was also obtained by calculation and verified by experiment. The results show that when the aging temperature was below 750 ℃, the α phase growth rate increased slowly, while the temperature higher than 800 ℃, the growth rate increased rapidly with temperature rising, as the element diffusion coefficient increased with the aging temperature rising. The α phase growth rate obtained by observing the microstructure of TC21 titanium alloy after isothermal aged at 900 ℃ was consistent with the simulated result. The α phase growth rate increased with the increase of aging temperature.