The high-speed melt spinning of sheath/core bicomponent fibers such as HDPE(11)/PET, HDPE(28)/PET and LMPET/HMPET was carried out and the tensile behavior of the as-spun fibers was investigated. In the case of HDPE/PET bicomponent fibers, the MFR(Melt Flow Rate) of HDPE component in bicomponent fiber did not influence the tensile properties of the two HDPE/PET bicomponent fibers, while the PET component in bicomponent fiber played an important role in determining the tensile behavior. In the case of LMPET/HMPET bicomponent fiber, the tensile properties somewhat differed from those of HDPE/PET bicomponent fibers. It appears the interfacial bonding in HDPE/PET bicomponent fiber is stronger bonding than in LMPET/HMPET bicomponent fiber. Three dimensional model analysis of the bicomponent fiber, assuming transverse isotropy and perfect bonding at the interface of the two components, was conducted and the distribution of stress and strain in each bicomponent fiber was calculated at low longitudinal strain. The distribution of radial stress and strain was predicted from the model analysis. The values calculated from the equations were also compared with experimental data and then the analysis was performed. From these results, we could predict some problems which may possibly occur at the interface of the two components and it appears that voids and poor interfacial bonding were important factors responsible for the different tensile behavior between HDPE/PET and LMPET/HMPET bicomponent fibers.