To investigate scattering characteristics of expanded graphite in 3 millimeter waves, with method of moment formule of radar cross section (RCS) are established based on electromagnetic scattering of finite length and conductivity cylinder. Average RCS of expanded graphite and its jamming decoy are calcutated. RCS of graphite and chaff jamming decoy are measured through field experiment. It shows that expanded graphite have some scattering ability in 3 millimeter wave. Average RCS of single expanded graphite particle is 0.867 6 mm² as its radius and length are 0.12 mm and 1.5 ram, respectively. RCS of chaff is 7.4 times of expanded graphite as interfering material mass are same. Expanded graphite is difficult to implement in the style of chaff decoy deception jamming on guided weapons.