Membrane property and feed characteristics play critical roles in membrane fouling. This paper aims to clarify the roles of membrane pore size (φ) and feed foulant concentration (C b) in ultrafiltration fouling induced by polysaccharides. The fouling behaviors were expounded by collision-attachment theory, where the rate of membrane fouling is mainly determined by collision frequency (JC b) and attachment efficiency (γ). At the initial fouling stage, rapid flux decline was observed at large φ or high C b due to the great JC b and/or γ. At the later fouling stage, there existed a nearly identical maximum stable flux attributing to the same JC b and γ, which was independent of φ and C b. Moreover, the smaller φ can lead to less foulants passed through the membrane and thus more foulants attaching on the membrane, while the higher C b can give rise to more foulants on both the membrane surface and in the permeate. The results presented in current study provide fundamental basis in understanding membrane fouling.