On the basis of analyzing expanded mechanism of expansible graphite,attenuation performance of expanded graphite in three-dimensional space at 3 mm and 8 mm wave was studied by experiments. The results show that expanded graphite has a valuable attenuation performance at 3 mm and 8 mm wave,and its attenuation performance decreases with the reduction of expansible graphite particle size. It is essential to choose appropriate particle size to obtain a valuable attenuation performance when expansible graphite is used as a new millimeter wave interfering material or the main component of smoke composition.