Disclosed herein is a method of preparing ZSM-5, including: providing a nanocrystalline ZSM-5 seed having a size of 70˜300 nm; adding the nanocrystalline ZSM-5 seed to a stock solution including water glass as a silica source, an alumina source, a neutralizer and water to form a reaction mixture; and maintaining the reaction mixture at 150˜200° C. to crystallize the reaction mixture. The method is advantageous in that ZSM-5 having small and uniform crystal sizes and including no impurities can be synthesized in a short period of time.