The tensile strength of fiberglass reinforced thermoplastics reached the maximum when the content of fiberglass treated by coupling agent was 30 %. The physical properties were improved as the alkali content in fiberglass was decreased. For the surface treatment of fiberglass the optimum amount of coupling agent turned out to be 0.01 % for both low-density polyethylene and high-density polyethylene and 0.05 % for polypropylene. It was also noticed that the addition of fiberglass caused unfavorable effects on physical properties when the surface was not treated by coupling agent. Concerning the mechanism of adhesion, the coupling agent was understood to adsorb on fiberglass, then form the hydrogen bond or hydroxyl bond with resins, and there by enhance the adhesive strength. This argument was supported by the Infra-red spectrum.