PURPOSE: Polyester undrawn yarns are drawn under specific conditions, then crimped in a stuffing box at a specific temperature, thereby producing title fibers showing self-stretching properties over 130°C and having high crimping performance.

CONSTITUTION: Undrawn polyester yarns are drawn at a temperature T°C ranging from 45°C to the glass transition point of the polyester in a draw ratio satisfying the equation where n is optical birefringence and DR is draw ratio. Then the resulting yarns are crimped in a stuffing box at 80W130°C. The yarns does not show self-stretching properties on the dyeing process and develops them on the heat setting process.