Regulations on the Norwegian Continental Shelf (NCS) are getting stricter with regard to discharge quality of produced water. As a mixture of dispersed oil in water emulsions, dissolved organic compounds, and inorganic particles, produced water could be treated with membrane separation. This method gives high effective separation of oil from water, yet still with fouling as the major challenge to succeed. In this paper the possibility of ultrasonic time domain reflectometry (UTDR) in explaining the mechanism of fouling which occurs during crossflow microfiltration (MF) due to the effect of particle size. was investigated. The flat sheet MF membrane with nominal pore size of 0.45 mu m was operated with cross-flow velocity of 4.16 cm/s and transmembrane pressure of 0.75 bar. It was observed that the cake layer followed by adsorption mechanism dominated when the particle size distribution was above the nominal membrane pore size, whereas the adsorption and compaction occurred in the range below and above the membrane pore size. 2D and 3D visualization technique was able to clarify the fouling mechanism as the effect of different particle size.