High-speed melt spinning of sheath-core type bicomponent fibers was performed and the change of fiber structure with increasing take-up velocity was investigated. Three kinds of polymers, poly(ethylene terephthalate)(PET), high-density polyethylene(MFR=11)(HDPE11) and high-density polyethylene(MFR=28)(HDPE28), were selected and two sets sheath/core combinations, HDPE(11)/PET and HDPE(28)/PET(i.e. sheath/core), were studied. The structure of each component in high-speed spun bicomponent fibers was analyzed through the birefringence, wide-angle X-ray diffraction measurements and DSC thermogram measurements. The development of molecular orientation and the initiating of the orientation-induced crystallization of PET in HDPE/PET bicomponent fibers occurred at lower take-up velocities regardless of the type of HDPE as compared to those in the single component spinning. The fiber structure formation of HDPE component in HDPE/PET bicomponent fibers was suppressed as compared to that in the single component spinning. The molecular orientation of HDPE in HDPE/PET bicomponent fibers was increased initially and then decreased with take-up velocities.