To investigate the fiber formation behavior of polyarylate TLCP(Thermotropic Liquid Crystal Polymer) fiber in the melt spinning process, we conducted on-line measurements of diameter under various spinning conditions in this study. It has been reported that polyarylate TLCP resin, which is polymerized with hydroxybenzoic acid(HBA) and hydroxynaphthoic acid(HNA), show typical shear thinning behavior in molten state and its highly oriented fiber is able to be obtained in melt spinning process. High degree of molecular orientation can be expected to let the fiber have high strength and high modulus properties. On the other hand, shear rate at nozzle and draft between spinning nozzle and take-up apparatus in melt spinning process can be expected to act as a main factor to obtain high orientation structure of fiber. However, there are little systematic studies to investigate the correlation with the fiber development behavior of polyarylate TLCP and the two factors using real time on-line diameter measurement in the spinning process. The on-line diameter measurements were conducted under the various spinning conditions using two auto-trackable optical sensors with 1.6kHz of high resolution. TLCP resin, polymerized with HBA and HNA at a ratio of 73:27, was supplied from Seyang Polymer Corporation in Korea. The resin was dried prior to the melt spinning, and melted by single screw type extruder(Φ25). The molten polymer was extruded through spinneret by gear pump. Throughput rate(shear rate) and take-up speed(draft) were controlled with 0.8~2g/min/hole and 500~2500m/min, respectively, at the two different spinning temperatures of 290 and 300℃. The on-line diameter measurements indicated that fiber diameter decrease drastically after extruding from spinning nozzle, and fiber deformation completed within approximately 400mm from the spinneret. The fiber deformation range distance between spinneret and solidification position show a tendency to decrease as the draft increase.