Studied was the effect of a high content of sodium sulphate in cooking liquor when three types of NSSC high-yield pulps were prepared from two species of hardwood chips. The three types of NSSC high-yield pulps were NSSC (series I) pulp substitute for USP with 65% yield ca, NSSC (series II) pulp for corrugating medium with 75% yield ca, and NSSC (series III) pulp substitute for GP with 85% yield ca. The chips were commercial-sized chips A (moisture 8.86%), chips B (moisture 11.84%), and pulp mill chips. Initially, pre-examinations (conventional NSSC process) were conducted under the conditions that cooking liquor for NSSC (series I, II, and III) was prepared from sodium sulphite and/or sodium carbonate, the chips were cooked (liquor ratio 5l/kg), and the cooked chips were treated with a disk refiner. Next, sodium sulphate was added to the cooking liquor (as Na2O) in accordance with its mol ratio of 6.25, 12.5, 25, or 100%, and then the cooking liquors were used for the NSSC cooking process. Under all conditions, the NSSC pulp was prepared without complications, and the pulp yield slightly increased. Strength of the NSSC pulp (I) and (III) slightly increased; however, the NSSC pulp (II) grade slightly decreased. When adding sodium sulphate of a mol ratio of 100, the NSSC pulp yield was 70.9% (CSF, 600ml), density 0.54, breaking length 4.1 km, burst factor 2.2, and tear factor 64. The NSSC pulp (II) yield was 77.5% (CSF, 464ml), density 0.68, breaking length 2.81 km, and tear factor 65. The NSSC pulp (III) yield was 85.0% (CSF 190 ml), density 0.62, breaking length 4.19 km, burst factor 2.37, and tear factor 57, displaying greater strength than conventional GP made from softwood.