Rice hull and straw were oxidized at temperatures in supercritical and subcritical water region to produce acetic acid by wet oxidation.Results showed that a high concentration of acetic acid may be obtained by controlling oxidation conditions. The highest acetic acid yield was about 14 % (TOC / TOC) of the starting materials, which is almost the same as that in the case of oxidation of food wastes. It was also found that there exists a large amount of other low molecular weigh carboxylic acids than acetic acid as residual intermediate products. It seems that the additional yield of acetic acid is hardly obtained from further oxidation of residual low molecular weigh carboxylic acids since the concentration of formic and oxalic acids, which are incapable of producing acetic acid, is relatively high among them. However, the purity of acetic acid in the solution after reaction can be increased easily by increasing the reaction temperature or reaction time, because other residual low molecular weigh carboxylic acids were oxidized much more easily than acetic acid.No significant difference in residual intermediate products and TOC decomposition was found when changing the reaction temperature from subcritical to supercritical water region over the critical point of water.