The role of glassfiber in reinforced plastics composite has been experimentally investigated. High density polyethylene composites filled with glassfiber of two forms were used for the studies. Continuous filaments and short fiber were experimented. The results indicated that the tensile strength of high density polyethylene composite filled with continuous glassfiber was in agreement with Hollister''s theory, but those filled with short parallel glassfiber showed only from one third to one half of the theoretically predicated strength values. When the surface of short glassfiber was pretreated by coupling agents, in the plotted range of volume fraction up to 15%, the strength was higher than the theoretical value.