In order to optimize the cellulase (from Aspergillus glaucus) hydrolysis of pretreated rice straw, the effects of varying enzyme concentration, temperature, and pH were studied. The best experimental conditions found to degrade the pretreated rice straws were 24 h of incubation at 55 °C and pH 5.0, with an enzyme concentration of 48 mg/L. Urea is one of the important nitrogen sources used in fungi culture, but it is also a denaturant. The model of denaturation of endoglucanase (EG) in urea solutions was established. The denaturation was a slow, reversible reaction. Determination of microscopic rate constants showed k(+0) > k'(+0), indicating that EG was protected by the substrate to a certain extent during denaturation. Comparison with the results from fluorescence emission spectroscopy revealed that the inactivation of EG occurred before the marked conformational changes could be detected.