In the last years membrane filtration emerged as an attractive technology for the treatment of oily wastewater, especially, as polishing step after extensive pre-treatment. We assume that this kind of treated oily wastewater is mainly composed of oil droplets in the range of nanometres which are able to plug the pores of porous membranes and thus cause severe fouling. This paper investigates crucial filtration parameters (such as the droplet size distribution, oil concentration and chemical composition) and fouling mechanisms while filtrating oil/water emulsion made from crude oil. For this purpose, stable emulsions with nano-sized oil droplets but different droplet size distributions were produced and filtrated through flat sheet polymeric MF and UF membranes in lab scale filtration units. It could clearly be shown that emulsions with oil droplets in the size range of membrane pores could cause higher initial fouling rate in comparison to emulsions where these droplets were missing. Furthermore, we found different removal ratios of UF for different components of the crude oil. Moreover, it was noticed that oil removal by MF increased with filtration time, which was not seen for UF membranes.