In contrast to conventional reservoirs, gas shales have very low porosity and low permeability (in nano-darcy range), and shale matrix possesses ruleless pore structure and a wide pore size distribution with a significant pore volume in the nano-pore range. The nano-pore structure of gas shale plays an important role in hydrocarbon storage and transport. The low-pressure gas adsorption and high-pressure mercury intrusion techniques are adopted for nano-pore structure characterization of the Upper Triassic Xujiahe Formation in the Sichuan Basin. According to the results, the nano-pore geometry of shale samples is relatively complex, mainly slit-shaped. The pore size distribution suggests multi-modal with a broad peak between 2 nm and 30 nm, and the pore volume is predominantly occupied by meso-pores (2~50 nm) and the main specific surface area is dominated by the micro-pores (＜2 nm) and meso-pores ( ＜50 nm). Characterization of nano-pore structure is of great importance for the percolation mechanism and reservoir evaluation study of shale gas.