Factors affecting the stability of three kinds of cellulose peroxides prepared by reactions of aldehyde cellulose, carboxy-methyl cellulose, and unmodified cellulose with hydrogen peroxide, Peroxides A, B, and C, respectively, were investigated. The relative stability of the peroxides was in the order Peroxide A > B > C, which was greatly improved by applying ethylenediaminetetraacetic acid as the chelating reagent for the metals in a very small amount in the sample. The sample kept under nitrogen retained stable activity of the peroxide longer than that under air atmosphere. Water in the sample seemed to participate in the decomposition of the peroxide to some extent. The functions of metals, water, and atmosphere on the decomposition of peroxide are discussed.