Abstract: The interfacial shear strength between various treated glass fibers and polypropylene was investigated. It was discovered that the main factor improved the interfacial shear strength was polypropylene grafted with maleic anhydride and the silane coupling agents had the less influence on it. That was due to the chemical reactions between bare glass fibers and grafted polypropylene which were verified by analysis. It was also found that the highest interfacial shear strength values of glass fiber and polypropylene could be obtained by using A 174 TM , not A 1100 TM which was usually used, to treat glass fiber when grafted polypropylene existed at the interface.