A liquid isothermal bath (LIE) was used in the spinline to produce high performance poly(ethylene terephthalate) (PET) fibres at high speed melt spinning in a one step process. The take-up stress was measured under various spinning conditions. It was found that maximum applicable take-up stress was determined by the state or morphology of the filament before entering the liquid bath. To render the filament more amenable to a preferred morphological texture prior to its entry into the liquid bath, a small amount of hot liquid was applied to the running filament at a position upstream of the liquid bath. The results show that rapid cooling leads to a non-uniform fibre structure and the loss of subsequent filament deformability. However, the prolongation of attenuation, attendant the heating of the filament before entering the liquid bath, induced uniform radial structure and an increase of deformability. Then the maximum operable liquid depth within the LIE could be attained. Furthermore, the delay of attenuation in the upper portion of the liquid bath and the attendant increase of stress within the liquid bath also resulted in an increase of the tensile properties. The modified LIE process was suggested as a further control of the temperature profile upstream of the liquid bath. High tenacity (9.7 g d(-1)), high modulus (>120 g(-1)) and high load at a specified elongation of 5% (LASE-5) (>4 g d(-1)) PET of as-spun fibres are achievable using the modified LIE one step process. The as-spun fibres produced with the modified LIE process have high amorphous orientation, low crystallinity and relatively large crystallite size. After drawing and annealing, the fibres exhibit desirable characteristics of high tenacity (>11 g d(-1)), high modulus (>140 g d(-1)), high LASE-5 (5.8 g d(-1)), and low shrinkage (<6%).