The microstructure evolution was analysised by heat treatment process at different solution temperatures and different cooling rates above or below βtransform temperature. The microstructure parameters such as grain size, total α phase area, thicknesses of lamellar α phase, primary α phase area and secondary α phase area were measured by software Image Proplus. The results show that for the heat treatment above βtransform temperature with furnace cooling, the grain size increases with the increase of temperature, its growth is bigger than that under BC cooling condition at the same solution temperature. During the heat treatment below βtransform temperature, for BC cooling condition, the total α phase area increases with increasing of temperature (800-850 °C), while it decreases as result of the secondary α phase disappearance (after 900-930 °C solution treatment). Moreover, for FC cooling condition (900-930 °C), the total α phase area increases as result of the largely precipitation of feathery lamellar α phase. The evolution?mechanism of?the grain?and the?volume fraction of?α phase?were analysised by the dynamics?theory.