In order to explore the effects of ultrafine grinding on the composition, structure, physicochemical and functional properties of dietary fiber from black rice bran, this study used black rice bran dietary fiber as the raw material to analyze the changes in the particle size, microstructure, adsorption characteristics and antioxidant capacity of the dietary fibers from ultra-fine and coarse black rice bran powders. The results showed that, the particle size and relative crystallinity of insoluble dietary fiber (IDF) and soluble dietary fiber (SDF) of black rice bran could be significantly reduced by ultrafine grinding, which led to a looser and more porous state, though their main compositions did not change. After ultrafine grinding, the swelling capacity, cholic acid adsorption capacity and cholesterol adsorption capacity of IDF from the black rice bran powder were increased by 57.43%, 22.71% and 24.92%, respectively, while the DPPH free radical scavenging capacity and FRAP antioxidant capacity of SDF were increased by 2.77 times and 1.71 times, respectively, those of the crude SDF. In conclusion, ultrafine grinding can significantly reduce the particle size of black rice bran dietary fiber, promote the formation of loose porous microstructure, and improve the physicochemical and functional properties of IDF. The research results provide a theoretical basis for the deep processing and utilization of black rice bran dietary fiber.