Coagulation prior to membrane filtration is widely accepted as a standard hybrid process because it allows poorer quality source waters to be used with less fouling. But more must be understood about the effect of coagulation conditions on the performance of membrane system, especially in submerged membrane system. The objectives of this research were to elucidate the relation between coagulation pretreatment and membrane performance and explore the effect of coagulation pretreatment and microfiltration on GAC post-treatment. Experiments were conducted in pilot scale using a 500 m3/day plant with submerged microfilters. Raw water collected from Han River was used as feed water and ploy aluminium chloride was used as coagulants. Granular activated carbon at EBCT of 15 min was used as post- treatment of membrane system to control taste and odor in product water. The results indicate that the coagulation condition had a significant impact on membrane fouling. Coagulation pretreatment was essential to stabilize transmembrane pressure (TMP) especially when the turbidity of feed water was high. As more organic matter was removed by coagulation, membranes fouled less. On the other hand, TMP was not sensitive to feed turbidity after coagulation and sedimentation. It appears that not only the removal of particles and organics but also the characteristics of coagulation flocs affect the membrane performance and pretreatment efficiency. Based on the experimental results, a technique using fouling index was suggested to control coagulation process. Coagulation was found to prolong the lifetime of GAC post-treatment by reducing dissolved organic matters up to 60 %. Includes 11 references, table, figures.