The particle-packing structure of Al2O3 slurry was evaluated by using constant pressure filtration. The influences of initial solid concentration and dispersant amount on the cake forming behavior were discussed. It was shown that the packing fraction of formed cake is almost independent of filtration pressure under 10 vol% of initial solid concentration. However at a relatively high solid concentration (20 and 35 vol%), the packing fraction of cake increased with filtration pressure. The cake forming behavior was classified into four patterns by plotting the resistance of the formed cake as a function of particle volume in the formed cake; the plots were (a) a straight line, (b) a convex curve, (c) a concave curve, and (d) a combination of a concave curve and a flat line. The particle dispersion and flocculate degree were discussed on the basis of the results obtained by constant-pressure filtration.