The millimeter wave attenuation performance of expanded graphite at 3mm and 8mm wavelength was studied by static experiments. The comparative experiment using aluminum chaff, which is a kind of well-known millimeter wave countermeasure material, was also done. The results show that the attenuation of expanded graphite at 3mm and 8mm wavelength are 17.0dB and 16.6dB respectively, and the attenuation of aluminum chaff are 7.4dB and 4.1dB. It means that the expanded graphite could be a better millimeter wave countermeasure material. The optimal particle size(including diameter and length) of expansive graphite that achieves the maximal attenuation was also obtained by experiments. These results could be a valuable reference for the design of full band screen smoke.