High speed melt spinning involves crystallization due to molecular orientation and subsequent necking in the course of spinning. In order to elucidate these phenomena, some experiments were carried out and simulation of high speed spinning of poly(ethyleneterephthalate) was done introducing related equations and assumptions. The results are as follows: 1. Elongational viscosity increases rapidly by the crystallization due to molecular orientation. 2. Necking in plastic deformation unfolds tie molecule. Necking stress is influenced by heating temperature. The higher the temperature, the lower the stress. 3. Annealing of super drawn PET increases degree of crystallization and birefringence. Degree of crystallization was rather influenced by annealing temperature than draw ratio. 4. Skin-core structure of high speed spun PET yarn was elucidated by the difference of diffusion coefficient of a disperse dye. 5. Molecular orientation induced crystallization in high speed spinning was simulated by using theoretical equations and introducing some assumptions. The results confirmed the radial distribution of relative crystallinity and the decrease of necking stress with increasing temperature.