PURPOSE: The titled fiber free from the lowering of elastic modulus at high temperature and suitable as tire cord, carpet, etc. is characterized in that the temperature corresponding to its peak value [(tanδ)max] are lower than the respective specific levels, and the Δn of the fiber is smaller than a specific value.

CONSTITUTION: The objective nylon 66 fiber is characterized in that the temperature (Tmax) corresponding to the peak of the mechanical loss tangent at the frequency of 110Hz is ≤100°C, the peak value (tanδ)max is ≤0.06, preferably ≤0.055, and the average birefringence at the fiber core [Δn(0)] is ≥6.0×10-2. Preferably, the extended molecular chain ratio (fs) in the fiber is ≥0.15, and the storage elastic modulus at 150°C (E150') is ≥5(GPa). The fiber can be manufactured by taking up the fiber at a spinning speed of ≥5,000/min, locally heating the fiber near the melting point under a tension of 0.06TW0.99T (T is tensile strength, etc.) g/d, quenching at ≤30°C, and again heating at 150W240°C.