The chemical and morphological properties of untreated rice straw and the straw pretreated with 0.5 M Na2CO3at 100◦C for 3 h (at which the best hydrolysis yields was obtained) were analyzed by FTIR spectroscopy. The spectra of untreated and treated straw showed the typical pattern expected for rice straw (Fig. 1).The absorption bands at 1430 and 896 cm−1 indicate the crystalline cellulose type I and type II, respectively. Cellulose type I is more resistant to enzymatic hydrolysis compare to cellulose type II (Poornejad et al., 2013). Total crystallinity index (TCI) and crystallinity index (CI), corresponded to the absorbance ratio of A1375/A2900 and A1430/A896 (Poornejad et al., 2013) were calculated from the spectra. These values were obtained to be 1.26 and 1.58 for untreated straw and 1.18 and 1.54 for treated straw,respectively. Therefore, the reduction of both TCI and CI of the straw presents the crystallinity destruction of cellulose which consequently improved the hydrolysis and fermentation of the straw.