PURPOSE: To obtain polyamide fibers having a high tenacity and improved thermal dimensional stability, by extruding a polymer containing poly(tetramethylene adipamide) as a main component and an antioxidant, e.g. a copper salt, under specific conditions.

CONSTITUTION: A polymer, containing repeating units consisting of 90% or more poly(tetramethylene adipamide), having a high polymerization degree as high as ≥3.0 relative viscosity measured in sulfuric acid, and containing one or more copper salts, e.g. cuprous iodide, and/or an inorganic or organic antioxidant other than the copper salts is molten within 305W325°C temperature range and then extruded through a spinneret nozzle at ≤1.5g/min throughput to give the aimed polyamide fibers having characteristics of formula II (T is the tensile strength), formula III (ΔS is the dry shrinkage at 150°C) and formula IV (Em is the elongation under 5.4g/denier load).