Bicomponent melt-spinning is one of the most widely used processes to produce high functional and special fibers. In recent, many studies have been reported that the structure development of each polymer components was enhanced or suppressed in bicomponent spinning systems, unlike the structure development in single-component spinning systems. The structure development of bicomponent fibers was strongly influenced by spinning conditions; such as spinning velocity or temperature, as well as surrounding environments; quenching air, polymer used, or interfacial area. Especially, island-in-the-sea type bicomponent fibers (SI fibers), in which a numerous fibers (island) are placed in a matrix (sea), have a large of interfacial area between island and sea components. Therefore, we expected that various properties of SI fibers can be more controlled and modified than other fibers. Herein, we prepared the SI fibers consisting of polypropylene (PP); sea component, and high-molecular weight poly(ethylene terephthalate) (HMW-PET); island component, by high-speed melt-spinning process. The structure developments of the SI fibers were evaluated by DSC, 2D-WAXD and mechanical properties, compared with single-component fibers. As results of structure analysis, we confirmed that the structure development of PP and HMW-PET components was suppressed and enhanced in SI fibers and the mechanical properties of HMW-PET were significantly maximized by SI type bicomponent fiber spinning systems. Furthermore, we fabricated the composite of SI fibers and single-component fibers and the mechanical strength and fracture behaviors were evaluated by UTM and SEM. The composite of SI fibers have highly integrated structure with strong interfacial adhesion and the higher mechanical strength than the composite of PP single-component fiber and blend of PP and HMW-PET single-component fibers.