The micro-structure of RDX was analyzed by small angle X-ray scattering (SAXS). The amount,scale and size distribution of pores in RDX were obtained by SAXS analysis. Results show that the magnitude of pores in the RDX samples decrease with increasing of the scales of RDX. Compared to the sample (<180 μm),the pores of the sample (>420 μm) decrease to about only a half. But the characteristic dimension of pores seems only have 1-2 nm difference. The Porod analysis of RDX shows that there are not only micro-voids but also cracks structure in the RDX.