Sludge dewatering is a common practice in many wastewater treatment plants. At the beginning of the cake filtration, particles smaller than the openings of filter medium pass through the medium. However, adhesion or trapping of solids occurs as the filtrate containing fine particles flows through the porous filter medium. The principal role of a filter medium is to separate solids from flowing liquid with a minimum consumption of energy. The particle size distribution of sludges were measured using Malvern Mastersizer, which was based on laser diffraction. The median values in particle size distribution were 20.75㎛ and 40.25㎛ respectively for the anaerobically digested sludge and the waste activated sludge used in this study. Increases of the medium resistance in according to the cake mass were found from the permeability test. The results were well explained by Tiller`s empirical equation including the medium resistance constant and the medium clogging constant. Total resistance in cake filtration was also measured with the permeability test and the average specific resistance was calculated from the slope of the regression result. The proper selection of filter medium will cause to decrease the energy consumption for the biological sludge dewatering.