This study explains the role of coagulation in membrane filtration of wastewater for reuse. For this purpose, a coagulation-ultrafiltration (UF) membrane system was used to treat secondary effluent from a nearby wastewater treatment plant using a rotating biological contactor. The study proceeded with the hypothesis that coagulation could affect membrane filtration through two phenomena: change in particle characteristics and contaminant loading reduction. If fouling reduction were observed at a low alum dosage, coagulation would affect membrane performance by changing particle characteristics because contaminant reduction could not be possible at low dosage. If fouling reduction were observed only at a high alum dosage, the role of coagulation would be contaminant loading reduction. Results showed that both phenomena were important. Coagulation improved the membrane performance by changing particle characteristics at a low alum dosage. The improvement was achieved through both a change in particle characteristics and contaminant loading reduction at a high alum dosage. Particle size among various characteristics was found the most important for membrane fouling. Coagulation increased particle size, which led to a reduction of fouling. The beneficial effect from coagulation was observed at both fouling steps of pore blocking/adsorption and cake formation. Coagulation pretreatment was also beneficial for the improvement of the permeate quality in terms of organic matter.