The effects of HMX crystalline inter voids, particle size, morphology and defects type on shock sensitivity were studied using standard gap test method. The shock initiation threshold of HMX/rap oil (76/24, wt.%) composition was detected with gap test where HMX was recrystallized to various qualities . Results show that the gap thickness decreases from 15.5 mm to 13.2 mm when the crystal apparent density increases from 1.8992 g·cm^-3 to 1.9016 g·cm^-3, and gap thickness increased from 13.0 mm to 14.3 mm with particle size increasing from 20 μm to 650 μm. The crystal defects type is major factors affecting shock sensitivity, and the shock sensitivities of HMX with twins defects is higher than that of without twin defects. The crystal morphology has almost no effects on shock sensitivity, when the crystal apparent density and particle size is same.