Since it has been made possible to injection-molded plastic enveloping wormwheels monolithically, contributions of such products to many industrial fields are expected. However, since strength and other properties of injection-molded plastic enveloping wormwheels are so far little known, there is pressing need to clarify them. Experiments for durabilities were conducted in this work with using three samples in total, namely, monolithically injection-molded enveloping wormwheels made of Nylon 66 prepared by reinforcement glass fibers which improve high-temperature durability, glass fibers and inorganic materials, and unreinforced. Basing on the experimental results obtained, examinations were made of the effects of the reinforcing glass fibers on the load-carrying characteristics and wear resistance.