Abstract: 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.gtoreq.-5.times.10.sup.-3 to 0.times.10.sup.-3, (b) long period (Dm) in the direction of fiber axis: Dm.gtoreq.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..gtoreq.125.degree. C., and preferably further satisfying the requirements: (d) birefringence .DELTA.n.gtoreq.60.times.10.sup.-3, (e) crystal orientation function fc.gtoreq.0.