Effects of RDX crystal characteristics including crystal internal porosity,particle size and distribution on shock sensitivities were studied by applying a method that RDX particles immersed in rap oil which excluded effects on shock sensitivity of binder/crystal interface,binder porosity and crystal shape. The shock initiation threshold of RDX/rap oil (mass ratio 76/24) compostion was determind with gap test where RDX was recrystallized to various qualities. Results show that the gap thickness decreases from 14.0 mm to 12.2 mm when the crystal apparent density increases from 1.7961 g·cm^-3 to 1.7983 g·cm^-3,and gap thickness decreases from 13.5 mm to 12.3 mm with increasing of particle size at the range of 50-600 μm,and this shows that crystal apparent density and particle size are two important factors affecting shock sensitivity. Impact sensitivity of RDX powder by drop weight apparatus was also studied. The test shows that drop weight impact sensitivity decreases from 75 cm to 35.8 cm evidently when particle average size increases from 16.7 μm to 337.9 μm,and does not almost vary while crystal apparent density changing at 1.7961-1.7983 g·cm^-3,which demonstrates that particle size is the important factor affecting impact sensitivity.