Microplastics (MPs) have been considered as a global environmental problem threatening the ecological security. However, studies on MPs' pollution in freshwaters and the associated risk assessment remain limited in the literature. In this study, the concentrations, distributions, and the potential ecological risks of MPs were analyzed in Yongjiang River, which is an important drinking water source flowing through Nanning City, the mega city of China. The MPs' abundances in surface waters and sediments ranged from 500 to 7700 n/m(3) and from 90 to 550 n/kg, respectively. Spatial distribution highlighted the significant impact of anthropogenic activity on the MPs' accumulation. Polyethylene and polypropylene were the most common polymer compositions investigated. Shape, size, and color were examined to analyze the characteristics of MPs in the river. To assess the ecological risk of MPs, the predicted no-effect concentration (PNEC) values were derived from a species' sensitivity distribution model based on the toxicity data of MPs for freshwater species available in the literature. The PNEC for MPs in surface water was derived to be 4920 n/m(3). Risk assessment results through risk quotient (RQ) method suggest that most of the monitored sites in Yongjiang River posed negligible risks to freshwater biota, except the two sites with high risk in the urban center. The results provided a basis for ecological risk assessment of MPs in freshwaters.