We focused on the colloidal contribution of lipid and protein in soymilk, and investigated the influence of pH on the stability of soymilk. The particle size distribution and viscosity were evaluated in relation to decreases in the pH of soymilk. By the addition of ascorbic acid, the average size of particles in soymilk was slightly increased at pH 6.0- 5.8 and remarkably increased at pH 5.6. Moreover, the fluidity index was severely reduced from 1 at pH5.8 or less. These results show that the viscosity of soymilk increased as a result of lipid and protein aggregation depending on the pH decrease. Then, soymilk colloidal stability was assessed using the centrifugal method. The state of soymilk changed in three steps as a function of decreasing pH. With respect to the viscosity and stability changes of soymilk, it was suggested that lipid-protein aggregates were generated, and the soymilk constituent content influenced the generation of lipid-protein aggregates. These results show that the pH-dependent stability of soymilk could be predicted from the constituent composition of soymilk.