Modified fine cellulose fibers are produced by impregnating a cellulose with a reactive fibrillation solution or mixture containing a catalyst including a base catalyst or an organic acid catalyst, a monobasic carboxylic anhydride, and an aprotic solvent having a donor number of not less than 26 to esterify and chemically fibrillate the cellulose. This method provides a simple efficient process for producing modified fine cellulose fibers that have a diameter from several nano-meters to submicrometers, a large aspect ratio, a high degree of crystallinity, less damage in the shape or crystalline structure of the fine fibers, a large aspect ratio, and an excellent dispersibility in an organic solvent; The catalyst may contain a pyridine compound. The monobasic carboxylic anhydride may be a C2-4aliphatic monocarboxylic anhydride. The resulting modified fine cellulose fibers modified with a monobasic carboxylic anhydride may have a degree of crystallinity of not less than 70%, an average fiber diameter of 20 to 800 nm, and an average fiber length of 1 to 200 μm.