Effect of Morphology and quantity for the primary α phase for TC4-DT titanium alloy on fracture toughness is discussed in this paper. Different α phase contents were obtained by heat treated below phase transition point. The result shows that contents of primary α phase drop with solution temperature increasing, and the size of grains change little. The content of secondary alpha phase increases, and length-width radio increases with solution temperature increasing, which leads to fracture toughness to increase. Sizes of primary α phase increase under furnace cooling than under air cooling, the thickness of lamellar α increase, fracture toughness of the alloy increases obviously, and the growth mechanism is analyzed through fracture.