In this study, a new method to form resin droplets on fibers has been developed, and samples for the single fiber pull-out test were prepared using this method. The effects of microstructure of polypropylene (PP) resin and the microstructure of interface between the glass fiber and PP resin on the interfacial strength have been investigated. In addition, the influence of the microstructure of the interface on the interfacial strength of glass fiber-reinforced PP composites have been discussed. It has been found that in the pull-out test, the transcrystallinity formed at the interface between the glass fiber and PP resin improved the interfacial strength when no spherulites developed in the PP matrix. On the other hand, it has been found that when the spherulites were well developed in the PP matrix, the transcrystallinity formed at the interface reduced the interfacial strength. Finally, rapid cooling has been shown to improve the interfacial strength between the fiber and resin in the crystalline polymer matrix composites.