Shales deposited in marine - continental transitional environments are well developed in the Upper Paleozoic in China with abundant gas resources. The pore structure and its controlling factors of the transitional shales may differ compared to marine and continental shales. Therefore, understanding them is of importance to the gas production behavior from transitional shales. In this work, a detailed experimental program combining rock composition analyses of X-ray diffraction (XRD), total organic carbon (TOC) analysis and Rock-Eval pyrolysis, and pore structure characterization by field emission scanning electron microscopy (FE-SEM), and low temperature nitrogen and carbon dioxide adsorption/desorption analysis were conducted on 16 continuously sampled shale cores from the east margin of the Ordos Basin, China. The results show that these transitional shales are rich in clay minerals with an average value of 58.2% (ranging from 47.7% to 85.0%), which is much higher than that in marine and lacustrine shales. The TOC contents are averaged at 1.3 wt%, and the shales are generally in mature to high mature stage (vitrinite reflectance (R-o) between 1.26% and 1.43%). Unlike the marine and lacustrine shales, the pores in the transitional shale are mainly occurred in clay minerals, and the organic matter pores are relatively poor and isolated. The pore shapes are dominantly intra-particle and inter-particle pores of slit-shaped as identified from the FE-SEM observations and the nitrogen adsorption/desorption curves. The meso (2-50 nm) and macro pore (generally of 50-300 nm) size distributions are unimodal to bimodal, and micropores (< 2 nm) vary between 0.4 and 0.85 nm. The micropores and mesopores are abundant for all the shale samples, accounting for more than 90% of the total pore volume and specific surface area. The surface fractal dimension D-1 and space structure fractal dimension D-2 were obtained from the nitrogen adsorption data. The D-1 values show no obvious relationship with clay minerals, TOC and quartz contents, which means that the pore surface irregularity is similar for all the samples. The D-2 values show a positive relationship with clay mineral content, showing the clay minerals directly influencing the pore structure and pore diameters heterogeneity. For successful development of transitional shales gas, a further investigation on the clay mineral composition are necessary, with an emphasis on its influence on well drilling and hydraulic fracturing.