Response surface methodology (RSM) was employed to optimize alkaline pretreatment condition for maximum glucose yield of rice straw with respect to NaOH concentration (1.0-4.0%), reaction temperature (60-100 degrees C) and pretreatment time (30-90 min). The maximum glucose yield of 254.5 +/- 1.2 g kg(-1) biomass was obtained at the desirable pretreatment condition of 2.96%, 81.79 degrees C and 56.66 min. The ANOVA test revealed that the model and all independent parameters were considered statistically significant at 95% confidence level. Furthermore, we tried to explore the effect of ultrasonication on alkaline pretreatment. The sample treated with both NaOH and ultrasound at high power showed a slightly higher digestibility yield than the sample with NaOH at 80 degrees C, but the difference was not significant. (C) 2012 Elsevier Ltd. All rights reserved.