A mathematic model is proposed for predicting the dynamic properties of fiber reinforced plastic composites (FRP) with interphase. Dynamic properties mainly include loss factors and elastic moduli. The results of elastic moduli from the present model are compared with other models. Elastic moduli E 11 , E 22 , G 12 , G 23 and loss factors g 11 , g 22 , g 12 and g 23 of FRP are calculated and a study of the effect of elastic modulus E I and loss factor g I of interphase on dynamic properties in FRP is carried out here. In this paper, E 11 linearly increases with E I , but the growth rate is slow. E 22 , G 12 , G 23 increase rapidly for lower value of E I . The longitudinal loss factor g 11 decreases with the increase of E I in the case of g I < 0.0026. But g 11 linearly increases with E I in larger range of g I , g I P 0.0026. On the contrary, transverse loss factor g 22 , transverse shear loss factors g 23 and longitudinal shear loss factors g 12 increase with increase of E I in the case of lower value of g I , but decreases at larger value of g I . And they also show insensitivity to E I at higher value.