Expanded graphite (EG) was prepared using fine flake graphite that could go through 50 mesh as raw material, perchloric acid as inserting agent and concentrated nitric acid as oxidant. The effects of amount of raw materials, reaction time and reaction temperature on expanded volume and sorption capacity of EG were studied, and microstructure of EG were characterized by X-ray diffraction patterns and scanning electron microscope. The results show the best process conditions are as follows: m(flake graphite):m(perchloric acid):m(concentrated nitric acid)=1:6:0.18, the reaction time is 50 min and the reaction temperature is 35°C. The reaction condition is moderate and is controlled easily. The sorption capacity of EG will increase almost linearly with increase of expanded volume. The largest expanded volume and sorption capacity of EG are 241 ml/g and 89g/g, respectively, when the expanding temperature is 800°C.