Abstract: ZSM-5 zeolites were synthesized from diatomite by hydrothermal crystallization method. The resulting samples were characterized by X-ray diffraction, FT-IR and scanning electron microscopy. The preparation factors were investigated such as crystallization time and temperature, pH of the synthesis system and the amount of NaAlO2. The results show that the optimal synthesis conditions are: 170 ℃ for 48 h in a solution with a pH value of 13.0 and the amount of NaAlO2 is about 4% of purified diatomite. The possible formation process of the ZSM-5 zeolite from diatomite during the hydrothermal crystallization is put forward based on the XRD and SEM analysis.