In a superdrawing process, a polyethylene terephthalate (PET) filament is elongated without developing much orientation and crystallization. Exploiting this phenomenon may bring about lower cost, more flexible and faster response in synthetic fiber production. The concurrent longitudinal and circumferential superdrawing phenomenon of PET hollow fibers is explained using the viscoelastic behavior of a thick walled cylinder under an internal pressure and an axial load in a continuous process. The model defines the stress-strain-displacement relationship of hollow fibers. The fiber undergoes instantaneous radial superdrawing (increase in thickness) in the process zone followed by concurrent circumferential (increase in void) and longitudinal (increase in length) superdrawing. Based on material viscoelastic properties and processing conditions, the model predicts the threadline tension, internal pressure, and final fiber geometries. Excellent agreement of the model with experimental results is observed over a range of processing conditions. The model is developed from a process engineering viewpoint to enable the analysis of the impact of process parameters during superdrawing on fiber properties. POLYM. ENG. SCI., 50:1773-1779, 2010. (C) 2010 Society of Plastics Engineers