Abstract: The microscopic pore structure has direct effect on gas bearing property of shale. Field emission scanning electron microscope (FE-SEM)was applied to describe pore morphology and types qualitatively of Longmaxi shale samples in south Sichuan Basin. Low temperature Ar adsorption experiment was carried out to measure the specific surface area (SSA), pore volume (PV) and pore size distribution (PSD) of the shale samples, and the continuous measurement of nanopore less than 100 nm was achieved. Fractal characteristics of nanopore structure were also studied by use of Frenkel-Halsey-Hill (FHH) model, and the relationships among total organic carbon content, pore structure parameters and fractal dimensions were discussed. The result shows that organic pores, intergranular pores and intragranular pores are developed in Longmaxi shale, and organic pores are dominated. The nanopores are slit shape on the Ar adsorption isotherms, and are mainly distributed in micropores and mesopores less than 10 nm, and the micropore size is mainly 0.6-0.9 nm and 1.8-2.0 nm, while the mesopore size is mainly 4.0-5.0 nm. The fractal dimension of nanopores ranges from 2.55 to 2.64, showing strong heterogeneity. Total organic carbon content controls the development of nanopores in shales. With the increase of TOC content, the number and proportion of micropores increase, and the fractal dimension also increases. All these factors complicate the pore structure of shale samples, and enhance the gas adsorption capacity of shale reservoirs. The research results have important significance for the study of nanopore structure of Longmaxi shale reservoir in southern Sichuan Basin.