The performance of high tenacity nylon 6.6 tyre cords with different linear densities (dtex) was investigated by measuring some mechanical, structural and physical properties. The structural changes due to the dipping process were analyzed by differential scanning calorimetry and X-ray diffraction analysis. The breaking strengths were increased with the increasing linear densities, but decreased with twist level. As the linear density increases, shrinkage values of greige cords were increased. Shrinkage is mostly dependent on amorphous parts of the cords. Since amorphous parts of the dipped cords are lower than greige cords, shrinkage values for dipped cord were lower than that of greige cords. Adhesion values were increased with the increasing linear density and twist level. It has been observed that increasing of the twist levels explicitly improved the fatigue resistance in the cases 1400 and 1880 dtex cords. But, no significant changes were observed for 940 dtex cords. It has been found that cords with lower linear densities had better fatigue resistance than that of the cords with higher linear densities.