Hydrogen peroxide has been a bleaching chemical for varied pulp, especially mechanical and deinking pulp. It is catalytically decomposed by some transition metals in pulp slurry. In this paper, some metals which can be contained in pulp such as manganese, copper, iron, magnesium and calcium were used to investigate their effect on the decomposition of hydrogen peroxide. From the result, hydrogen peroxide was more decomposed in the order of Mn, Cu, $Fe^{3+};and;Fe^{2+}$, while Mg and Ca had little effect on the decomposition of hydrogen peroxide. The effect of Mg/Mn ratio on the decomposition of hydrogen peroxide was also investigated. At the specific ratio of them(Mg/Mn=10), hindering effect of peroxide decomposition by Mg was decreased.