Despite the great advance in nano-filtration (NF) process development in various applications, its performance in water treatment is still impeded by the undesired fouling phenomenon, which is not yet fully understood nowadays. In this study, a comprehensive investigation on the fouling behavior, more specifically the combined fouling behavior with the presence of more than one type of foulant was carried out under various solution chemistries and feed hydraulic pressures. The interaction between the model organic foulant (alginate) and inorganic foulant (silica colloid) was systematically studied with nano-filtration membranes, with a more severe fouling observed in the combined fouling than individual foulants. However, the results also revealed that the commonly observed synergistic effect of combined fouling in the forward osmosis process was not present in the nano-filtration process. In addition, the solution chemistries (pH and calcium ions) and feed hydraulic pressure were also found to have certain influence on the combined fouling, especially the feed hydraulic pressure which exhibited a more pronounced impact on the flux decline than the other factors, thus in turn, affecting more on the reversibility of the membrane fouling.