at 177 u00b0 c in heat set process the polyethylene terephthalate cords are also treated with rfl dipping solution and adhesion with the rubber in tire is provided the glass transition temperature of polyethylene terephthalate is higher than that of nylon 6 6 which causes less flatspot in the tire the polyethylene terephthalate cords comprising the reinforcement strips used in the inventive pneumatic radial tire 1 have a secant modulus value in range of 2 0 2 5 mn dtex between 3 5 elongations at 160 u00b0 c in other words the said values represent the slope of the line for 1 elongation combining a and b points in fig1 linear cord density is gram value of 10 000 length of a yarns in cord dtex elongation range 5 3 is the elongation value in the inventive pneumatic radial tire 1 the thermal shrinkage force is increased above 2 0 mn dtex at 177 u00b0 c in order to increase the pressure of the cords on the belt package at high speed and temperature a modulus over 2 5 mn dtex may cause over stretched cords during production process of the tire since a modulus below 2 0 mn dtex will also cause a decrease in thermal shrinkage force a.