A process was investigated for producing high modulus, high strength, dimensionally stable poly(ethylene terephthalate) (PET) filament by combination of threadline modification and post drawing techniques. Unlike traditional commercial processes for the production of PET filament yarn, the present process uses a liquid isothermal bath to develop an extremely high level of tension in the threadline. This results in extremely high amorphous orientation and low crystallinity in the as-spun fibres, with good mechanical properties, similar to those found in products produced using a commercial spin-draw process. The oriented amorphous structure produced directly by such a modified spinning process can be further oriented by a subsequent hot-drawing operation, resulting in a final PET filament product which possesses ultrahigh birefringence (nn > 0.23) and excellent mechanical properties. The tenacity, elongation, and initial modulus (T/E/M) are, respectively, in the range of 9.5-10.7 g d(-1), 6-9%, and 130-150 g d(-1). Experimental filaments also exhibit excellent resistance to yielding within the range of small strain, with tensile stress values as high as 5-7 g d(-1) being observed at 5% elongation. This superior balance of properties, high modulus, high tenacity and high dimensional stability, is unprecedented in conventional commercial processes for the production of PET filament.