A new way of applying on-line experimental data and basic theory to study the mechanism of orientation of a high-speed melt spinning process is described. The relationship of birefringence and stress for Nylon-66 was developed to understand the phenomena in the spinning line. The value of birefringence along the spinning line was calculated by various models to predict the orientation change. By comparison of the model prediction and on-line experimental birefringence, a suitable mechanical model to simulate the change of the profiles along the spinning line was chosen, and the structural development mechanism is discussed. The results show that the orientation mechanism of high-speed melt spinning of Nylon-66 is determined by deformation and deformation rate along the spinning line. For Nylon-66, molecular and crystal orientations develop independently and are controlled by the rotation of crystals and chain segments in the deformation field.