To characterize the pore structure and quantify fractal dimensions of tight gas sandstones, a case study is performed on the Lower-Middle Permian tight sandstones in the Ordos Basin in China by conducting a series of experiments including X-ray diffraction (XRD) analysis, routine petrophysical measurements, thin section and scanning electronic microscope (SEM) observations, and nuclear magnetic resonance (NMR) experiment. The studied tight sandstones mainly consist of quartz and clay minerals, and pore types include primary intergranular pores, inter- and intragranular dissolution pores, as well as micropores associated with clay aggregates; T-2 spectra reflect three types of pore size distributions in the studied samples, indicating a rather irregular pore distribution pattern in tight sandstones; NMR can estimate porosity of tight sandstones accurately, and movable-fluid porosity from NMR can better reflect the permeability of tight sandstones than total porosity. Two fractal dimensions, D-bnd (with respect to bound-fluid pores) and D-mov, (with respect to movable-fluid pores), are calculated to be 1.1135-1.8116 (average 1.4750) and 2.6816-2.9932 (average 2.8921), respectively. D-bnd increases with the decrease of detrital quartz content and the increase of clay mineral content, whereas D-mov increases with the increase in authigenic quartz content and the decrease of detrital quartz content; fractal dimensions can reflect the physical properties of tight sandstones, as large D-bnd and D-mov values typically result in low movable-fluid porosity and permeability; the pore network of tight sandstones can be considered as a dual-scale pore system based on fractal theory, whereas D-bnd and D-mov can reveal the roughness of bound-fluid pore surface and the distribution of movable-fluid pores, respectively. This study shows that NMR fractal dimension can be employed as an effective indicator to characterize the pore network of tight sandstones.