Biomass was pretreated with proton beam irradiation (PBI) in order to enhance enzyme digestibility. Rice straw and soaking in aqueous ammonia (SAA)-treated rice straw were treated with 1-25 kGy doses of PBI at a beam energy of 45 MeV. The optimal doses of PBI for efficient sugar recovery were 15 and 3 kGy for rice straw and SAA-treated rice straw, respectively. When PBI was applied to rice straw at 15 kGy, the glucose conversion reached 68% of the theoretical maximum at 72 h. When 3 kGy of PBI was applied to SAA-treated rice straw, approximately 90% of the theoretical glucose conversion was obtained at 12 h compared to a 89% conversion at 48 h. After 2 h of enzymatic saccharification, the initial reaction rates of raw rice straw pretreated with 15 kGy of PBI and SAA-treated rice straw pretreated with 3 kGy of PBI were 1.4 x 10(-4) and 9.7 x 10(-4) g L-1 s(-1), respectively. Further, the results of X-ray diffractometry support the effect of PBI on sugar recovery, whereas scanning electron microscopy images revealed a more rugged rice straw surface.