Abstract: The physical basis of nondestructive testing for composite structures using the coin-tap method has been investigated. A typical criterion of inspecting defects is established. Meantime.the minimum detectable defect for GFRP composite laminates including different defects has been analyzed in detail. A defect of lOmm diameter under a 2mm thick laminate is at the margin of detectability. Therefore,the method is particularly attractive for use in the field and on laminate structures that may be porous.