The effects of spinning conditions on shape changes in the trilobal-shaped fiber spinning of poly(ethylene terephthalate) (PET) and polypropylene (PP) fibers were studied with a focus on spinning parameters such as mass flow rate, spinning temperature and take-up. Extrudate at the die-swell points along the spinline was obtained to investigate shape change. Extrudate shape at the die-swell points affected the shape factor of as-spun PP fibers. Surface tension affected the fiber cross-section of the PET trilobal fibers, whereas die swell was more important in the PP trilobal fibers. The shape factor of the PP trilobal fibers decreased with increasing mass flow rate, spinning temperature and take-up speed, whereas that of the PET trilobal fibers increased with increasing mass flow rate and take- up speed.