This study focuses on investigating the phenomena involved in the formation of the concentration polarization and fouling layer during the cross-flow ultrafiltration process of cellulose nanocrystal (CNC) suspensions. Thanks to new cross-flow ultrafiltration cells specially designed, experiments of first time-resolved in situ Small Angle X-ray Scattering (SAXS) and second time-resolved in situ micro Particle Image Velocimetry (micro-PIV) at the vicinity of the membrane surface during filtration were performed. These two methods have given access to the concentration profiles and the velocity field as a function of the distance z from the membrane surface within the concentration polarization layers. The results obtained show an increase in particles concentration related to a decrease in the velocity within the layers formed during the process. This information linked to the rheological behavior of the suspensions permitted access to the calculated stress field within the concentrated layers during the cross-flow ultrafiltration process. Three different areas of shear stress/shear rate behaviors near the membrane surface have been emphasized. The important role of the rheological behavior and interparticle interaction in the dynamic evolution in space and time of the accumulated layers has been highlighted.