On-line measurement was performed in the high-speed spinning of fiat, hollow, and circular fibers of poly(ethylene terephthalate), paying particular attention to the change in cross-sectional shape along the spin line. The diameter profiles of hollow and circular fibers were essentially identical, whereas the deformation of flat fiber shifted to the region closer to the spinneret. The necklike deformation of hollow and circular fibers started at the takeup velocity of 5 km/min. In the case of fiat fibers, presence of the necklike deformation was confirmed at 4 km/min, and extremely steep diameter attenuation was observed at 5 km/min. The spin-line tension of the fiat fiber was also larger than that of circular fibers. Combined measurements of fiber velocity and thickness enabled us to evaluate the aspect ratio of the flat fiber and hollow ratio of the hollow fiber in the spin line. These two factors were found to decrease steeply near the spinneret. Accordingly, the thinning of the spin line and the change in cross-sectional shape appeared to proceed independently.