This study investigates effects of ultrasound (US) on cross how ultrafiltration of skim milk by multi scale characterization, using a custom designed "SAXS Cross Flow US coupled Filtration Cell". The study of flow properties of casein micelle suspensions shows an evolution of their rheological behavior from Newtonian to shear thinning until the emergence of yield stress with the increase of concentration (from 27g L-1 to 216g L-1). The concentration profiles during cross flow filtration of skim milk have been revealed for the first time by real time in-situ Small Angle X-ray Scattering (SAXS) measurement. Without any change of internal structure of casein micelles and membrane selectivity, the applied ultrasound (20 kHz, 2 W cm(-2)) leads to a significant increase of permeate flux arising from a disruption of concentrated layer. Varying the US intensity from 0.6 W cm(-2) to 2.9 W cm(-2) does not affect the US enhancement factor, which however depends on the feed concentration. In fact, increase of feed concentration induces the formation of highly cohesive fouling layer during filtration that the applied US could hardly disrupt. Results also suggest that the preventive US application mode is promising since formation of the reversible fouling layer was strongly limited in this mode. (C) 2014 Elsevier B.V. All rights reserved.