The dynamic properties of composite materials consisting of an ethylene-propylene rubber matrix (EPDM) and short polyester polyethylene-terephthalate (PET) fiber vary with dynamic stress amplitude applied to the material. These variations support the statement that fiber treatment with 1,4-carboxy-sulphonyl-diazide, which acts as a bridge between the fiber and the matrix and hence enhances the strength of the interface enabling it to resist greater strain applied to the composite and, as a consequence, yielding greater retention values of the storage modulus, measured longitudinally to preferential fiber orientation, E’(L). By means of transversal measurements of the storage modulus, E’(T), of these materials it is possible to determine a parameter b, which eventually indicates the degree of matrix-fiber bonding and which is consistently higher for materials filled with surface-treated fiber. This enhanced phase adhesion is further confirmed by higher equivalent interfacial thickness values, Delta R, which, in addition, vary less with increasing dynamic strain amplitude. Finally dissipated energy variation or mechanical energy loss, E(loss), is studied as a function of fiber content and strain amplitude. Experimental findings show E(loss) to increase with fiber content and strain amplitude, when measured at constant strain amplitude epsilon(0), and to yield higher values for treated fiber samples.