The main objective of this study was to investigate the fouling mechanism of various types of ultrafiltration membranes with different pore sizes by the cross-flow filtration of biological suspensions. The cross flow experiments were conducted using two different membrane types (cellulose-UC- and polyethersulfone-UP-) with three different molecular weight cut off (MWCO) (5, 10, 30 kDa for UC and 5, 10, 20 kDa for UP). The most fouling was observed in UC030 membrane for which the initial flux and the final flux values were 205 L/m2/h and 89 L/m2/h, respectively. Higher porosity caused greater initial flux that transported colloids and SMP fractions to the surface filing up the pores or pore openings and causing more fouling. Almost no drop was observed in flux values of the membranes of UC005, UC005 and UP010, indicating that almost no fouling were occurred for these membranes. This was a result of the accumulation of foulants in the pores or in the pore openings. As MWCO increased, higher membrane flux was observed, on the other hand, lower SMP rejections were achieved. UC membrane with MWCO of 30 kDa showed the most rapid flux decline among all membranes which was attributed to its irregular and rough surface structure.