A method according to the present disclosure may includes preparing a urea solution by dissolving urea in distilled water, adding phosphoric acid to the urea solution, adding pulp to the solution in which urea and phosphoric acid are dissolved, heating the solution such that the urea and the phosphoric acid each react with the pulp and preparing nanocellulose by washing the pulp which is completely reacted, and then grinding the pulp, in which a weight of the phosphoric acid is 10 to 50% based on a weight of the pulp.