Flat fibers and hollow fibers were prepared through the high-speed melt spinning of poly(ethlylene terephthalate) (PET), and the structures of these fibers were compared with those of circular fibers. The cross-sectional shape of each fiber changed to a dull shape in comparison with that of the respective spinning nozzle. The change in the cross-sectional shape: was slightly suppressed with an increase in the take-up velocity. There was a significant development of structural variation in the cross section of flat fibers in that the molecular orientation and crystallization mere enhanced at the edge. Despite the difference in the cross-sectional shape, the structural development of flat, hollow, and circular fibers with increasing take-up velocity showed almost similar behavior. Considering that the tensile stress at the solidification point of the spin line is known to govern the structure development of high-speed spun PET fibers, it was speculated that the effects: of the enhancement of cooling and air friction on the tensile stress at the solidification point cancel each other.