Membrane filtration has been increasingly used in water purification and wastewater treatment and reuse. However, membrane fouling is still a major problem to the membrane filtration process. Membrane fouling occurs in the boundary layer at the membrane surface where impurities rejected by the membrane accumulate and form a dynamic fouling layer. Owing to the lacking of experimental means, this dynamic fouling process that has not been well characterized. In this study, the micro-particle image velocimetry (MicroPIV) technique was utilized to achieve in-situ investigation of the membrane fouling dynamics near the membrane surface during the crossflow ultrafiltration in a flat-sheet membrane cell. A membrane sheet was placed on the bottom of the crossflow filtration cell. Fluorescent latex nanoparticles sizing 600 nm were used as particulate impurities or tracers in the feed solution. When excited by laser, the nanoparticles can emit fluorescent light. A high-speed scientific CCD camera equipped with a microscopic objective lens was employed to take photos of the flow field over the membrane surface during the ultrafiltration process, while the fluorescent signals from the nanoparticles were captured by the camera. With the MicroPIV technique, the dynamic process of fouling layer formation was directly observed. By further image processing, the thickness and the concentration of the dynamic fouling layer were well determined.