The dynamic fracture toughness of TC4 titanium alloy in 7 heat-treatment conditions, such as 750 oC, (20-60) oC lower than or 10 oC higher than the phase transformation temperature, was tested by means of instrumented impact method. Combined with both metallography and fractographs, the influence of volume of primary α phase and ratio of length/width for secondary α phase was studied. Results show that the volume of primary α phase in 47%-50% with short rod-like secondary α phase leads to relatively good dynamic fracture toughness for TC4 titanium alloy with (primary α+βTrans) microstructure under the dynamic loading condition.