Twisting has an influence on not only the strength, luster, softness, and elasticity of the yarns, but also the properties of the resulting fabrics. This study aims to evaluate how the rotor twister speed and the thermal treatment temperature influence the properties of the polyethylene terephthalate (PET) plied yarns and wrap yarns. Both yarn types are made with a twist coefficient of 1, 1.5, or 2, and then thermally treated in an oven at 70, 110, and 150 ° C The resulting plied yarns are wrap yarns are tested for tensile strength and elongation, and their surface is also observed by a stereomicroscope to determine the influences of the two parameters. The results of the experiment show that twist coefficient does not influence the tensile strength nor the elongation. The tensile strength of various plied yarns and wrap yarns is approximately 4.4cN/dtex; the variation in the twist coefficients is too small and thus hardly causes any significant differences in the yarn's properties.