It is well known that the interaction and adhesion between the glass fiber (GF) and polymer matrix has a significant effect in determining the properties of fiber-reinforced materials. Therefore, it is one of important considerations to modify the surface of glass fiber with an appropriate sizing. We investigated the treatment method of glass fiber with coupling agent to improve the interaction of the interfacial region. The correlation between interfacial property and interphase microstructure was also examined in an attempt to realize a proper morphology at the glass fiber surfaces.