The present investigation deals with the mechanical, thermal and viscoelastic properties of ternary composites based on low density polyethylene (LDPE)-ethylene-propylene-diene terpolymer (EPDM) blend and high density polyethylene (HDPE)-EPDM blend reinforced with short jute fibers. For all the untreated and compatibilizer treated composites, the variation of mechanical and viscoelastic properties as a function of fiber loading (10, 20 and 30 wt %) and compatibilizer concentration (1, 2, and 3%) were evaluated. The flexural strength, flexural modulus, impact strength, and hardness increased with increasing both the fiber loading and the compatibilizer dose. The storage modulus (E) and loss modulus (E") of the HDPEEPDM/jute fiber composites were recorded higher compared to those of the LDPE-EPDM/jute fiber composites at all level of fiber loading and compatibilizer doses. The tan 8 (damping efficiency) spectra showed a strong influence of the fiber loading and compatibilizer dose on the a relaxation process of polymer matrix in the composite. The thermo-oxidative stability was significantly enhanced for treated composites compared to untreated composites. Scanning electron microscopy investigation confirmed that the higher values of mechanical and viscoelastic properties of the treated composites compared to untreated composites is caused by improvement of fiber-matrix adhesion as result of compatibilizer treatment. (c) 2008 Wiley Periodicals, Inc.