It is well known that, in the high-speed melt spinning of bicomponent fibers, structure development behaviours of individual components are different from those in the single-component spinning of respective components because of the kinematic mutual interaction of the two components in the spinning process. With the aim of obtaining the fibers of unique characteristics, sheath/core bicomponent fibers consisting of the low and high molecular weight poly(L-lactide)s (LowMw-PLLA and HighMw-PLLA) as the sheath and core components were prepared through the high-speed melt spinning process. Both PLLAs have the high optical purity with the D-lactide content of less than 0.5 %. The sheath/core ratio was set to 50/50. The attainable maximum take-up velocities, which are the measure of the spinnability, were 8, 9 and 10 km/min for the HighMw-PLLA, Sheat/core and LowMw-PLLA, respectively. Thinning and solidification behaviour of the spin-line analysed through the on-line measurement of the spin-line diameter indicated that the neck-like deformation started to occur from lower take-up velocity in the order of HighMw-PLLA, Sheath/core and LowMw-PLLA. Position of the neck-like deformation at the same take-up velocity was closer to the spinneret with the same order. Different fiber formation behaviour in terms of the starting of the orientation-induced crystallization between the three types of spinning was also confirmed from the Wide-angle X-ray Diffraction analysis of the as-spun fibers. After the starting of the orientation-induced crystallization of the sheath/core bicomponent fibers, the birefringence of the HighMw-PLLA component was higher than that of the single-component HighMw-PLLA fibers, whereas the birefringence of the LowMw-PLLA component was lower than that of the single component Low-Mw-PLLA. It is interesting to note that the birefringence of the LowMw-PLLA component decreased significantly with the increase of take-up velocity and reache