A high-tension annealing (HTA) method has been applied to nylon 66 fibres to improve their mechanical properties. The HTA was carried out three times under high tension close to the tensile strength at break. The nylon 66 fibre used for the HTA treatment was previously zone-drawn twice at 210 degrees C under applied tensions of 14.5 MPa and 182.2 MPa. To orient amorphous chains selectively, two HTA treatments were carried out; the first HTA (HTA 1) at 100 degrees C under an applied tension of 143.3 MPa, and the second (HTA2) treatment at 110 degrees C under 142.0 MPa. The third (HTA3) treatment was carried out at 190 degrees C under 75.1 MPa to crystallize the amorphous regions oriented by the HTA 1 and HTA2 treatments. The resulting HTA3 fibre had a draw ratio of 7.3, a value of birefringence of 0.0761, and a degree of crystallinity of 40.4%. The orientation factor of crystallites almost approached unity in the first high-temperature zone-drawing (HT-ZD1) stage, but that of amorphous regions (f(a)) was 0.575 in the HT-ZD1 stage. The f(a) increased with the processing and finally reached a high value of 0.852 for the HTA3 fibre. Dynamic moduli increased with the processing, and the dynamic modulus of the HTA3 fibre was 21.7 GPa at 25 degrees C and 15.0 GPa even at 200 degrees C.