The 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. In this study, poly (ethylene terephthalate) (PET) and two kinds of high-density polyethylene (HDPE (11), HDPE (28)) were selected for the core and sheath, respectively. In general, the birefringence of the sheath part of single component fiber is higher than that of the core part. But the birefringence of sheath component (ie. HDPE(11) and HDPE(28)) of the bicomponent fibers used in this study was lower than that of core component. To understand how the structure of bicomponent fiber was formed at high speed spinning, numerical simulation was performed. The results of numerical simulation showed that the spinline stress of core component was higher than that of the sheath component in the region of structure formation and the difference between the stresses at the sheath and core increased with increasing take-up velocity. The results of numerical simulation coincided with the birefringence experimentally obtained. When higher tension is given to the fiber, the results of numerical simulation coincided well with the birefringence profiles. It seems that the birefringence of PET increased due to the increasing difference of stress between PET and HDPE components.