Ultrafiltration (UF) is an efficient and effective method of filtrating oil-in-water (O/W) emulsions. However, the favorable conditions of filtrating nano-sized O/W emulsions have not been investigated. This study investigated the influence of four different parameters including membrane pore size, cross flow velocity, pH and salinity on membrane fouling as well as oil rejection. Alumina UF membranes were employed to filtrate nano-sized O/W emulsions. O/W emulsions were synthesized using soybean oil and stabilized by surfactants. The filtration experiments were conducted under constant flux for multi cycles. The results showed that 200nm was recommended to filtrate nano-sized O/W emulsions (average droplet size -100nm) based on the high oil rejection (95% ~ 99%) and low irreversible fouling resistance. Besides, alumina membranes were proved to be more effective in fouling mitigation at low salinity, high pH, and high cross flow velocities.