Dynamic moduli, E′ and E″, and loss tangent tan δ were investigated for thermoplastic elastomers (TPEs), styrene-isoprene-styrene copolymers (SISs), styrene-butadiene-styrene copolymer (SBS), and Hytrel and composites reinforced by poly(ethylene terephthalate) (PET) short fibers. The styrenic TPEs have a typical rubbery behavior and the Hytrel TPE has medial characteristics between rubber and plastic. Both E′ and E″ of the composites depended on the matrix as well as the fiber loading and fiber length. Based on the viewpoint of different extensibility between the fiber and the matrix elastomer, a triblock model was considered for estimating the storage modulus of the short fiber-TPE composites as follows: Ec = αVfEf + β (1 − Vf) Em, where α and β are the effective deformation coefficients for the fiber and the matrix elastomer, respectively. They can be quantitatively represented by modulus ratio M (= Em/Ef) and fiber length L: α = ( Ln + k) M/ ( LnM + k), β = (1 − α Vf)/ (1 − Vf), where the constants n and k are obtained experimentally. When k = 0.022 and n = 0.45, Ec of the TPE composites agreed well with the prediction of the proposed model. The relaxation spectrum of the composites showed a distinct main peak ascribed to the matrix elastomer, but no peak to the PET fiber. The relative damping of main relaxation, (tan δmax)c/(tan δmax)m, decreased monotonously with increasing fiber loading and fiber length and with decreasing modulus of matrix elastomer. Thus, the relative damping may be attributed not only to the volume effect of matrix, but also to the unevenness of the strain distribution in the matrix phase, which depends on the fiber length and the matrix's modulus. The findings prove that the different extensibility between fiber and matrix and the uneven distribution of strain in matrix were important for the short-fiber reinforcement of the TPE composites. © 1993 John Wiley & Sons, Inc.