Abstract: In order to reduce the viscosity of black liquor, a threefactor and three-level orthogonal experiment was employed to study the effects of alkali charge, maximum temperature and holding time at the maximum temperature on the viscosity of black liquor from wheat straw soda pulping by a simple heat treatment. While a fourfactor and threelevel orthogonal experiment was designed to study the effects of alkali charge, maximum temperature, holding time, and oxygen pressure on the viscosity reduction of black liquor from wheat straw soda pulping by an oxygen enhanced heat treatment. The results of variance analysis showed that the viscosity reduction of wheat straw black liquor by heat treatment with oxygen was much efficient than that of treated by simple heat treatment. The viscosity reduction was more prominent when the viscosity concentration more than 30 %. The critical concentration of black liquor treated by heat with oxygen, if compared with the similar heat treatment condition but without oxygen enhanced, increased by around 5 %. And the most remarkable factor which influences the viscosity reduction by heat treatment with oxygen was the maximum temperature. In addition, the optimal condition of heat treatment with oxygen was determined as follows: alkali charge on total solids content was 1.0 % (as NaOH), with maximum temperature of 170 ℃, holding time at the maximum temperature of 45 min and oxygen pressure of 0.7 MPa.