Carboxy-telechelic polyolefin prepolymers were synthesized via ring-opening metathesis polymerization of 3-ethyl-lcydooctene in the presence of maleic acid. Following hydrogenation these difunctional prepolymers were cross-linked with a trifunctional aziridine in the presence of fumed silica or carbon black fillers to yield polyolefin elastomers with significantly enhanced mechanical properties as compared to the unfilled versions. The tensile strength, modulus, and tear strength of the elastomers can be tuned by changing the filler type and the total filler content. Further study on the reinforcing mechanism suggests that both the polymer filler interactions and the filler filler interactions play key roles in the mechanical property enhancements.