Abstract: Effect of series heat treatment on microstructure and room temperature mechanical properties of TC4-DT alloy bar in diameter 300mm was investigated.The results show that in the microstructure of 300mm bar forged in α+β region,the grains are coarse and vague with obvious grain bindery in some field,which suggests that the big TC4-DT bars has poor uniform inforgings.The tensile properties and fracture toughness of big bars being treated after α+β double annealing can reach to R_m ≥ 825MPa,R_P0.2 ≥ 750 MPa,A ≥ 8%,Z ≥ 15%;K_IC(T-L) ≥ 90MPa·m^½,indicating excellent match properties between strength and plasticity.The fatigue crack propagation vacuity can reach 2.833036×10^-6mm/cycle and 7.294209×10^-6 mm/cycle respectively after heat treated at 965℃/1h AC+550℃/6h AC and 940℃/1h AC+570℃/6h AC heat treatments.