In this study, ply yam is used with the PET filament and dipped with RFL system. By changing the twist multiplier, we analyse the properties of strength, elongation and heat shrinkage for the PET ply yam. It is found the twist shrinkage and elongation increasing by twist multiplier, but the heat shrinkage decreasing. The best strength is 2 of twist multiplier in first twist and 3.5 of twist multiplier in second twisting of 2x5x l000d. The best strength is 2.5 of twist multiplier in first twist and 3 of twist multiplier in second twisting of 3x3xl000d. The other type of 6x3 x 1 000d, the best strength are 2 of twist multiplier in first and second twist.