Abstract: The effect of aeration on cellulase production by Trichoderma reesei was studied with 10g/L paper pulp. At 28℃, with the impeller speed of 250r/min, and pH value 4.8, the aeration was either unadjusted or adjusted in order to control dissolved oxygen tension (DOT). The experiments were carried in a 10-L steam sterilizable bioreactor, mycelium consistence and cellulase activity were mensurated. The results showed: when aeration was unadjusted, DOT was either below the critical DOT or too high, which was not favorable to the growth of mycelium. When DOT was unadjusted, the best value of mycelium consistence was 2.77g/L, the best value of filter paper enzyme activity (FPA) was 2.80IU/mL.By adjusting aeration, the DOT at 20%-30% was most favorable to the growth of mycelium, the best value of mycelium consistence reached 3.12g/L,which was 12.6% higher than that of unadjusted aeration, FPA reached 3.55IU/mL at 76h, which was 26.8% higher than that of unadjusted aeration.