In this study, PET (intrinsic viscosity of 1.05 dl/g) was melt processed with a horizontal isothermal bath (HIB) treatment. Tensile properties of PET fiber samples were highly increased by using the HIB. The process‐structure‐property relationship of control (no HIB) and HIB fiber samples were characterized by tensile testing, differential scanning calorimetry, birefringence measurement, scanning electron microscopy and hot‐air shrinkage measurements. It was found that HIB fiber samples, which had been subjected to post‐drawing process, had a high degree of molecular chain orientation, that is, a high birefringence, high crystallinity and a fibrillar structure. The best tensile property acquired from a HIB‐drawn PET fiber sample was 10.24 g/d in tenacity, 114.17 g/d in modulus, and 13.49% in elongation at break. Applying the HIB in the melt spinning process was simple and required only small process space; hence, it is cost effective. In addition, acquiring HIB fiber samples was successful at a final take‐up speed of 2,500 m/min. Hence, this HIB‐assisted melt spinning technology has a high potential to be used in industries for technical textiles applications. POLYM. ENG. SCI., 57:224–230, 2017. © 2016 Society of Plastics Engineers