In order to enhance the interfacial-adhesion between short E-glass fibers and low-density polyethylene (LDPE), acidic whey; a dairy byproduct was applied as coupling agent. The fibers were immersed in acidic whey for 12, 24, 36, 48 and 60 hours. Composite materials containing 20 wt.% E- glass fibers and LDPE were prepared by extrusion process. The tensile strength and modulus of the produced composites were improved by using acidic whey, and the improvement in those properties was strongly affected by the immersion time of E-glass fibers in acidic whey. An increase of about 289%, 107%, 291% and 721% in modulus of elasticity, tensile strength, modulus efficiency and strength efficiency factors respectively were achieved after 60 hours of immersion of glass fibers in acidic whey. The effects of acidic whey on tensile properties and efficiency factors could be ascribed to the mend in interfacial-adhesion between glass fibers and LDPE polymer.