A detailed study was conducted to investigate the effects of horizontal isothermal bath (hIB) on the production of ultra-high performance polypropylene filaments. Two different commercial PP polymers were used with the melt flow rate of 4.1 and 36 g/10 min. The optimum process conditions depended on polymer molecular weight. Fibers showed distinct precursor morphology for each at each optimum process condition. However, two sets of filaments demonstrated similar fiber tenacity and modulus of about 7 and 75 g d(-1), respectively, for as-spun and more than 12 g d(-1) for tenacity and more than 190 g d(-1) for modulus values of drawn fibers after just 1.49 draw ratio. The mean value for the modulus after the drawing process for the high melt flow rate was 196 g d(-1). The theoretical modulus of PP is 35-42 GPa(19), (275-330 g d(-1)), shows the hIB fiber's modulus performance is approaching its theoretical maximum value. Fibers had greatly improved thermal properties, degree of crystallinity, crystalline and amorphous orientation factors. The hIB spinning system produced highly oriented and predominantly amorphous structure for as-spun fibers and a well-defined, highly oriented crystalline fibrillar and amorphous structure after drawing process with the draw ratios lower than 1.5. POLYM. ENG. SCI., 55:327-339, 2015. (c) 2014 Society of Plastics Engineers