These studies were conducted to investigate the chemical composition changes in in vitro digestibility for the improvement of nutritive value of rice straw by alkaline hydrogen peroxide. The content of neutral detergent fiber (NDF), acid detergent fiber (ADF), hemicellulose, cellulose and lignin in rice straw was decreased with higher level of H₂O₂ (pH 11.5). The content of ADF, cellulose and ash of the rice straw washed after H₂O₂ (pH 11.5) treatment tended to be increased but NDF, hemicellulose and lignin were decreased with higher concentration of H₂O₂ (pH 11.5). In the rice straw washed after alkaline hydrogen peroxide treatment the decomposition of cellulose and lignin was effective in pH 11.5∼12.5, in smaller cutting size and 55℃. The in vitro organic matter digestibility was increased with higher H₂O₂ concentration and smaller cutting size of rice straw.