Exfoliated graphite is a potential military passive interfering material, which can attenuate infrared and millimeter-wave simultaneously. Exfoliated graphites with different expanding volumes were prepared by two-step intercalation, in which the mixture of nitric acid and phosphoric acid, nitric acid and acetic acid were employed as intercalating agent, and potassium permanganate was used as oxidant. The microstructures of exfoliated graphites with different expanding volumes were analyzed by scanning electron microscope. The millimeter-wave attenuation performances of exfoliated graphites with different expanding volumes were measured by a static method. The results show that the exfoliated graphites′ sheets become thinner with the increasing of expanding volume, the interlayers are fully opened and there are more tiny graphite flakes with size distribution between several microns and several ten microns. And the millimeter-wave attenuation performance enhances with the increasing of expanding volume. When the expanding volume increases from 233 mL/g to 450 mL/g, the 3 mm wave attenuation value rises from 5.7 dB to 8.68 dB.