The characteristics of compression and solid-liquid separation of excess activated sludge were investigated. Compression-permeability experiments were made under low solid compressive pressure (10(3)-10(5) Pa), and centrifugal settling experiments under 30-2000 Pa. In these experiments, we used excess activated sludge from a municipal wastewater treatment plant and sludge treated by a freezing and thawing process. An equation for inorganic sludge (1 - epsilon = E.p(s)beta) is proposed, where epsilon and p(s) are the porosity and the solid compressive pressure, and E and beta are empirical constants for each sludge. The equation can be applied to both experiments, if the average values of epsilon and p(s) which were defined previously by the authors are used in the centrifugal settling experiment. Consequently, the compression characteristics over a wide range of p(s) can be estimated by a centrifugal settling experiment which is relatively simple. The relationship between local specific filtration resistance alpha and p(s) can be represented by a straight line which has a bending point at the pressure where epsilon(w) is nearly equal to zero. The freezing and thawing treatment of excess activated sludge diminishes the specific filtration resistance of the sludge by more than one order over the whole experimental range of p(s). This implies that our treatment improves the solid-liquid separation characteristics of excess activated sludge.