Abstract: A micromechanics analysis on the interphase of fiber reinforced polymer compolite is presented. The representative volume element of coaxial combined cylinder model is adopted. Four compliance parameters S_ij(i,j=1, 2) are used do characterize the interphase effect, which generalizes Hashin's twoarameter interface mode. A relation between these parameters and macro behaviour of composite is established based on the equivalent principle of a multiphase body. By experiment,interfacial parameters are first determined for two kinds of composites:T300/ST 350 and Glass/Polyester.