Polyester fibers for rubber reinforcement and dipped cords using same

A polyester fiber containing polyethylene terephthalate at 90 mol% or higher of a whole repeating unit in a molecular chain thereof, the fiber having an intrinsic viscosity ÄIVÜ of 0.85 dl/g or higher and simultaneously meeting the following characteristics: (a) strength ≥ 6.0 cN/dtex; (b) strength x (breaking elongation) ≤ 26.0 cN/dtex.%; (c) monofilament linear density ≤ 5.0 dtex; and (d) main dispersion peak temperature of loss tangent (tan delta ) in the measurement of dynamic viscoelasticity at 110 Hz ≤ 147.0 DEG C, and a polyester dipped cord, which is obtainable by twisting one or more than one base yarn together into a pretwisted yarn, where the base yarn is made of a polyester fiber according to any one of claims 1 to 4; twisting two or more pretwisted yarns together into a greige cord; and subjecting the greige cord to dip treatment to give a dipped cord simultaneously meeting the following characteristics: (a) tenacity conversion efficiency in the dip treatment (dipped cord tenacity / greige cord tenacity) ≥ 96%; and (b) elongation at a specific load + dry heat shrinkage ≤ 7.5%.