To investigate the non-steady-state melt spinning behavior take-up velocity was varied periodically in the high-speed melt spinning process of polyethylene terephthalate and the time-course changes of velocity and diameter of the spin-line were measured simultaneously. A numerical simulation program for the non-steady-state melt spinning process was also developed. In the range of relatively low take-up velocities, experimental results and results of numerical simulation of the non-steady-state melt spinning process showed a fair agreement, in that the amplitude for the variation of the reciprocal of the cross-sectional area and that for the variation of velocity of the spin-line had a linear relation. An increase in the frequency of velocity oscillation led to a less significant variation in the thickness of as-spun fibers. On the other hand, there was a phase shift between the oscillation of take-up velocity and that of the reciprocal of cross-sectional area of the spin-line at the point of take-up. When take-up velocity was increased, starting of the orientation-induced crystallization along with the occurrence of neck-like deformation caused some characteristic behaviors such as significant increases in the amplitudes of velocity and diameter fluctuations at a certain region in the spin-line. Such behaviors could be reproduced in the numerical calculation by incorporating the stress dependent change of solidification temperature. It was also revealed that the orientation-induced crystallization has the effect of stabilizing the spinning system.