To increase the acetic acid yield in a wet oxidation of lignocellulosic wastes, a new two-step reaction process is proposed. The first step produces 5-hydroxymethyl-2-furaldehyde (HMF) and 2-furaldehyde (2-FA) by dehydration of monosaccharides which are formed by hydrolysis of polysaccharides under the condition of non-supply of oxygen. In the second step. HMF and 2-FA are oxidized to acetic acid with oxygen newly added. The yield increased about twofold.