Microfiltration of particle suspensions is widely used, especially in bioprocess engineering. However, particle accumulation at the filter surface reduces productivity and can modify selectivity of the separation. Numerous macroscopic models have been developed to predict filtration performance and help in device design, assuming a uniform cake formation. In order to improve the model accuracy, cake formation mechanisms have to be better understood at the microscale level. The objective of this work is to study the cake build up at the particle level and to establish correlations with microfiltration performance measured at the process scale. 3D characterization of microsieves fouling was achieved using Confocal Laser Scanning Microscopy (CLSM) and a filtration chamber allowing direct microscopic observations of particle deposition and cake characterization, specially designed for in situ observations. Suspensions of fluorescent monodispersed and bidispersed spherical particle (1 mu m and 4.8 mu m) were filtered under constant flow rate through 0.8 mu m pore diameters silicon nitride microsieve. Online images of particle deposition were recorded and the cake build up was followed layer by layer. The relations between cake resistance and particle organizations were studied.