Sulfate ester modified cellulose nanofibers having an average fiber diameter in the range of 1 nm to 500 nm, and having sulfate ester modified hydroxyl groups on surfaces of the cellulose nanofibers. A method of producing cellulose nanofibers that are nanosized, that have a high crystallinity degree, and that have large aspect ratios, the method being a chemical method that does not require any physical pulverization, that is energy-saving, and that can be performed under mild reaction conditions. A method of producing modified cellulose nanofibers including modifying the surfaces of the cellulose nanofibers through esterification or urethanization. A method of producing cellulose nanofibers includes impregnating cellulose with a fibrillation solution containing dimethylsulfoxide, at least one carboxylic acid anhydride selected from acetic anhydride and propionic anhydride, and sulfuric acid to fibrillate the cellulose.