In this study, to verify the characteristic behavior of fiber deformation in high-speed melt spinning process of high molecular weight poly(ethylene terephthalate) (IV 1.2), on-line measurements of fiber diameter were performed for the melt spinning line at the take-up velocities of 1 to 5 km/min by using 2 way (XY-axis) high-resolution optical measurement apparatus with sampling rate of maximum 16 kHz. Overall diameter profiles obtained in the spinning line showed that neck-like deformation accompanying strain-induced crystallization started to appear from around 4km/min and its position shifted toward the spinneret with an increase in the take-up velocity. Detailed analysis of neck-like deformation through high-resolution simultaneous diameter measurements at two XY positions of close distance confirmed that the position of necking deformation fluctuates along the spinning line and maximum strain rate varies significantly depending on the take-up velocity and the direction of its movement.