Abstract: To improve the high-value utilization of the coffee processing by-product, the coffee peel insoluble dietary fiber (CPIDF) was modified by wet ball milling, the effects of ball milling time on the composition, structure and functional properties of the CPIDF was investigated. The results showed that the soluble dietary fiber (SDF) content of CPIDF increased significantly (P<0.05), the particle size decreased and the absolute value of ζ-potential increased after treatment with wet ball milling. The microstructure of wet ball milling treated CPIDF particles showed more irregular surface pores and cracks, but without any changes of functional groups. With increasing the ball milling time, the suspension stability, water holding force, oil holding force, swelling force and emulsification capacity of CPIDF were improved. Compared with the untreated group (CPIDF-0), the water holding capacity of CPIDF treated with 12 h ball milling (CPIDF-12) increased from 7.00 g/g to 17.12 g/g, the oil holding capacity increased from 2.60 g/g to 9.64 g/g and swelling capacity increased from 3.91 mL/g to 6.90 mL/g. The emulsification index (EI) increased significantly from 38.85% (CPIDF-0) to 100% (CPIDF-12) (P<0.05). The emulsions prepared from CPIDF with more than 8 h ball milling treatment showed good stability at different temperatures, pH, ionic strength and storage time. Conclusively, the wet ball milling treatment improved the functional food properties of CPIDF which had the potential to be used as a food-grade solid emulsifier.