The ultrafiltration (UF) process has been applied to water purification for drinking water treatment. However application of membrane filtration is limited by the problem of irreversible fouling, especially in case of using surface water, showing high concentration of natural organic carbon (NOC), iron, and manganese. This study examined the effect of pretreatment by granular activated carbon (GAC) for membrane fouling. It is known that the pretreatment by GAC filter accumulate both biological and non-biological particles on the filter bed. And biological pretreatment is effective to remove biodegradable compounds? iron, and manganese which may be fouling matter on the surface of membrane. Thus, GAC filter was considered to play part of preventing irreversible fouling and minimize chemical cleaning frequency and to realize lower pressure operation. Experiments were completed by using surface water of Onga river (Japan), showing relatively high concentration of iron and manganese. Pilot testing showed the pretreatment decreased chemical cleaning frequency, and cellulose acetate membrane was most suitable for the river water. And other testing results demonstrated that the pretreatment by GAC and the cellulose acetate UF treatment brought stable operation for 530 days without chemical cleaning.