For UF/MF and NF membrane process for the drinking water treatment, algae have been known to directly or indirectly affect membrane filtration. During algae bloom, transmembrane pressure seriously increases or flux significantly decreases. Algae can also produce taste and odor causing compounds such as 2-methyl isoborneol (2-MIB) and geosmin as well as toxic materials. In this study, microcystis aeruginosa was evaluated as a representative algae with an UF membrane in terms of fouling mechanisms by algae. The size and surface charge of microcystis were 2.8 gm and -12.35 mV, respectively, as measured by electrophoresis light scattering method. 10 pm silica was used to serve as a secondary membrane which was expected to remove smaller particles (i.e., here microcystis) before they reach the membrane surface. However, silica did not effectively act as the secondary membrane for microcystis contrarily to expectation. Combination of microcystis and natural organic matter (NOM) provided higher flux decline than either of microcystis and NOM filtration with the UF (EW) membrane. Concentration polarization (CP) thickness of microcystis on the EW membrane was calculated by both convection-diffusionelectrophoretic migration (CDE) and convection-diffusion (CD) models with consideration of the negative charge of membrane and algae. The CP thickness estimated by the CDE model was higher than that by the CD model. And, microcystis fouled on the membrane surface could be more effectively cleaned by 100 ppm NaOCl solution than 0.025 N NaOH solution.