The evolutions of grain boundary orientation differential of TA15 alloy after annealing heat treatments at different temperatures were investigated by EBSD and TEM. The results show a few small-angle grain boundaries of TA15 alloy transformed into large-angle grain boundaries when annealed at 800 ℃; more grain boundaries with large angle appeared greatly while annealed at 950 ℃; the average thermal activation energy was 151.4 kJ/mol for the transformation from small-angle grain boundaries to large angle grain boundaries, which is equal to the self-diffusion activation energy of α-Ti; it means that the transformation from the small-angle grain boundaries to the large-angle boundaries was controlled by the self-diffusion of titanium atoms. TEM micrographs show sub grain boundaries transformed into dislocation net with hexagonal net at the annealing temperature of 800 ℃, while lots of equiaxed grains were obtained after annealing treatment at 950 ℃.