Abstract: T'he calculation of RGS for an arbitrarily shaped and perfectly conducting scatterer with small electric size has been studied by moment method.The starting point of this paper is the reaction integral equation（RIE）.We use polygonal plates to construct the models of realistic structures and segmemt every plate into modes automatically.Overlap modes can be obtained between the connectiag intersecting plates.The piecewise-sinusoidal modes have teen chosen as expansion and testing functions .From RIE, the matrix equation IZ=V can be derived. Accord-ing to 1,scattering field and RCS of the scatterer can be obtained. The method has advantages of simplicity and flexibility of input ,versatility and accuracy.The calculating results of RCS for a few scatterers with different shapes,including a realistic rocket,have been given.The results are in good agreement with those publishod abroad.