Ultra- and microfiltration of skim milk are widely used in the dairy industry as tools for the concentration and the fractionation of proteins but their performances are limited by formation of a concentrated layer of casein micelles at the membrane surface. Significant technological efforts are being made in order to reduce the concentrated layer formation and increase the permeate flux but there is a lack of knowledge on the behaviour of concentrated layer (mainly layer formation, sol-gel transition and reversibility) during cross-flow filtration. However some evidences of the possibly effect of operational conditions have been highlighted. Qu and co-workers showed indeed the effect of time and hydrodynamics conditions in the structural organization of casein micelles in concentrated layer respectively in static and dead-end filtration [1, 2]. Moreover, previous experiments [3] using Small-angle X-ray scattering (SAXS) showed the impact of strong shearing (the ultrasound) on cross-flow ultrafiltration of skim milk and allowed to give unique information about the concentrated layers in-situ. The aim of this work is to understand the properties of casein micelles accumulated layer under various operational conditions during cross-flow ultrafiltration. Combining SAXS (ID02 TRUSAXS beamline, ESRF) and cross-flow filtration in dedicated ultrafiltration cells, we studied the effect of time, shear and temperature on the structural organization of casein micelles during cross-flow filtration. Concentration profiles have been deduced from the scattering intensities registered in-situ in the course of the cross-flow filtration of casein micelles dispersions. Relaxation phases of layers formed under various operating conditions have been studied. SAXS and filtration results for a reference experiment (0.06L/min, 3h of filtration, ambient temperature) show some reversibility of the upper layer of accumulated casein micelles layers and an irreversibility of fraction closed to the membrane surface. A stronger shearing (0.1L/min instead of 0.06L/min), a shorter time of contact between casein micelles (1h50 instead of 3h) and a change of temperature (45°C or 12°C instead of 25°C) led to a different organization of concentrated layer and a thinner irreversible layer. Those results are important for understanding the concentrated layer properties of casein micelles during cross-flow filtration and they could be used to adjust the filtration parameters in order to avoid the formation of an irreversible concentrated layer at the membrane surface.