Disclosed is a method of preparing ZSM-5 having substantially 100% crystallinity by using variable temperatures in the absence of an organic template, characterized in that a reaction mixture having a molar composition of M.sub.2O/SiO.sub.2 (M: alkali metal ion) of 0.07-0.14, H.sub.2O/SiO.sub.2 of 15-42 and SiO.sub.2/Al.sub.2O.sub.3 of 20-100 is nucleated at relatively high temperatures (180-210.degree. C.) and then crystallized at relatively low temperatures (130-170.degree. C.), thus easily controlling a crystal size and a particle size distribution of the ZSM-5.