The formula of radar cross section (RCS) of chaff and the derived formulas are educed. Based on the moment method and the Galer Kin method, the differences between the induction current distributions of chaff scatting and Dipole radiation, and the far-field patterns are calculated and compared, and the influences of the attitude of chaff in space on induction current distributions and far-field patterns are discussed. Lastly, the definition and formulas of average effective RCS of one single chaff in the single-station and bi-station scattering conditions are given.