The reason for this behavior is attributed to the weak adhesion between the paper sludge and wood particles because of the inorganic materials, such as kaolin clay and calcium carbonate, in the paper sludge. Furthermore, paper sludge mainly contains short fibers that cause a decrease in the bending strength of the boards. The results also showed that bending strength of the UF-bonded boards decreased more sharply than that of the MDI-bonded boards. This is assumed to be due to the high reactivity of isocyanates with the inorganic materials of paper sludge (Pizzi, 1983 and Xiaoqun, 2003). The bending strengths of the UF-bonded boards containing 15% paper sludge were higher than those of MDI-bonded boards.