Abstract Porosity of a cake layer formed through particle deposition plays an important role in the filtration performance of low-pressure membranes. An experimental analytical method for the direct measurement of cake porosity in a coagulation–microfiltration was developed using a CLSM and image analysis technique. A series of aggregates suspensions with different sizes and fractal dimensions were prepared with 0.1 μm of fluorescent polystyrene latex beads. Microfiltration of each suspension was carried out to form a cake layer whose porosity was measured using CLSM with an optical section of 2 μm and an image analysis program. To validate this method, measured porosities were compared with those calculated from specific cake resistances using the Carman–Kozeny equation. The two sets of porosities were in good agreement, confirmed statistically by a t-test. Based on this analysis technique, the effects of size and fractal dimension of aggregates on cake porosity and compressibility were investigated under various operating pressures. For low fractal dimensions the cake porosity was barely dependent on the fractal dimension of aggregates, whereas for high fractal dimensions it decreased with increasing fractal dimension.