High-strength polyamide fiber

PCT No. PCT/JP94/00281 Sec. 371 Date Oct. 24, 1994 Sec. 102(e) Date Oct. 24, 1994 PCT Filed Feb. 23, 1994 PCT Pub. No. WO94/19517 PCT Pub. Date Sep. 1, 1994.To provide a polyamide fiber such as polyhexamethylene adipamide fiber having high tenacity and excellent tenacity retention after vulcanization and GY fatigue life. This fiber is a high tenacity polyamide fiber, preferably a polyhexamethylene adipamide fiber comprising at least 95% by mole of hexamethylene adipamide units and having a sulfuric acid relative viscosity of at least 3.0, said polyamide fiber satisfying the requirements: (a) differential birefringence delta DELTA n>/=-5x10-3 to 0x10-3, (b) long period (Dm) in the direction of fiber axis: Dm>/=105 A, and long period (De) in the radial direction of fiber: De=90 to 130 A, (c) main dispersion peak temperature (T alpha ) in mechanical loss tangent (tan delta ) curve as obtained by viscoelastic measurement: T alpha >/=125 DEG C., and preferably further satisfying the requirements: (d) birefringence DELTA n>/=60x10-3, (e) crystal orientation function fc>/=0.88, and (f) amorphous orientation function fa=0.70 to 0.85.