Background and Objectives: Cellulose is a major component of plant biomass, which can be converted into biofuels and valuable chemicals. The key step in utilization of this organic material is its hydrolysis into soluble sugars. This study evaluated cellulase production by Trichoderma harzianum under different pH values, temperatures and incubation periods with the aim to increase enzyme production and decrease its costs.Methods: The amount of protein production and the hydrolytic activity of cellulase enzymes including exoglucanase, endoglucanase and b-glucosidase produced by T.harzianum were evaluated using various substrates such as avicel, carboxymethyl cellulose, cellobiose, Whatman grade 1 filter paper under different pH values (4, 4.8, 5.5 and 6.5), temperatures (25, 28 and 34 °C) and incubation times (48, 72, 96 and 120 h).Results: The optimum condition for cellulase production by T. harzianum is 120 hours of incubation at 25 °C and pH of 6.5.Conclusion: T. harzianum can be used for the production of all three classes of cellulase. This fungus is suitable for the efficient production of cellulolytic enzymes and reducing the cost of consumables.