Effect of substrate size for the direct bioconversion of rice straw and bagasse to ethanol by Fusarium oxysporum was studied in a batch reactor at constant temperature and pH. Maximum ethanol concentration was obtained for 100 mesh size screen in eight days when the substrates were pre-treated with alkali. It was found that the ethanol concentration was higher for bagasse when compared to rice straw. Maximum ethanol concentration was obtained with a substrate concentration of 2% in 100 mesh size scree for bagasse and ricestraw were found to be 10.59 g/L and 8.6 g/L, respectively. The kinetic data were fitted with logistic growth model and Leudeking-piret model and it was found that the former model closely represents the fermentation process whereas Leudeking-piret model represents the fermentation process only at the initial stages.