Concerning the heat treatment of defatted soybean flour, the fitness of water dispersibility of protein as a criterion of the degree of denaturation has been investigated. Denaturation was measured by the change of susceptibility of the protein in water extract of the flour to protease action of Aspergillus sojae before and after heat treatment. It was found that the decrease of water dispersibility did not always run paralell with denaturation. The dispersibility method was applicable only to flours autoclaved for a definite period which varied according to temperature and the water content of the flour treated.Discussion is made about the fitness of measuring the degree of denaturation by protease action.