This work reported for the first time the cake structure of consolidated biological sludge in Compression-Permeability Cell (C-P cell) tests without the interference of sludge rebound after stress release. The filter cake equilibrated with the applied load was fixed in situ by agarose solution. Then the porosity of slices of cake was determined using image analysis tool. Both original and flocculated sludges were consolidated at a pressure of 128.3 kPa. The cake structure is nonhomogeneous. The applied load and the action of wall-cake friction squeeze the cake toward, and yield the lowest porosity at the rim region on the septum. The flocculated sludge cake has a higher porosity than the original sludge. However, flocculation could produce a more elastic cake, which results in a completely different cake structure in C-P cell test.