The effect of twist level on the mechanical and thermal properties of nylon 66 and polyethylene terephthalate (PET) tyre cords has been studied. Effects of the twist on some critical cord properties such as tensile properties, shrinkage, shrink force, adhesion and fatigue have been evaluated. Breaking strength was decreased between 3.1 and 7.3 twist factor values, whereas breaking elongation was increased, on both nylon 66 and polyester cords. The tensile behaviour of high twist factor PET is similar to that of low twist factor nylon cords. This is an advantage for the possibility to get closer the properties of different materials by adjusting theirs twist factors. The shrinkage values increase with increasing twist factor, whereas shrinks force values decrease for greige nylons and polyester cords. Adhesion and fatigue resistance is increased with increasing twist factors.