The consolidation behavior of nanometer-sized particles at 20-800 nm was examined using a pressure filtration apparatus at a constant compressive rate. The relation of applied pressure (Delta P-t)-volume of dehydrated filtrate (V-f) was compared with the established filtration theory for the well-dispersed suspension. The theory was effective in the early stage of the filtration but deviation between the experiment and the theory started when Delta P-t exceeded a critical pressure (Delta P-tc). It was found that this deviation is associated with the phase transition from a dispersed suspension to a flocculated suspension at Delta P-tc. The factors affecting Delta P-tc are the zeta potential, concentration, and size of the particles. Based on the colloidal phase transition, a new filtration theory was developed to explain the Delta P-t-h(t) (height of suspension) relation for a flocculated suspension. Good agreement was shown between the developed theory and experimental results.