The different level microstructure (grains,colonies and lamellas)were obtained when TC4 titanium alloy was heat-treated at different cooling rates, and the Hall-Petch relation was established based on different level microstructure. With the increase of cooling rate, the lamellas were refined; the yield strength and tensile strength increased, whereas the ductility gradually decreased.The relationship between lamella size and mechanical properties was in line with Hall-Petch relation, therefore the lamella was the main characteristic parameter affecting the mechanical propertiesand.