The factors determining the fiber aspect ratio (FAR) of PET flat yarns were studied in terms of various nozzle aspect ratios (NAR), spinning temperature, spinning speed, and melt flow rate, etc. Also the relations between NAR and tensile properties were investigated. Nozzles with ARs from 2 to 50 were used in these experiments. As a result, FAR was 2.5 and 10 when the NAR were 3.7 and 50, respectively. FAR increased with the decrease of spinning temperature, and the increase of spinning speed and melt flow rate. It was found that the tenacity decreased with the increase of FAR. After drawing (draw ratio=3.2), the FAR of drawn fiber in creased compared with the FAR of undrawn fiber.