A fouling index (FI) was introduced as a novel approach to investigate natural organic matter (NOM) fouling behavior in a low pressure membrane ultrafiltration process. Humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA), were used in the experiments. According to FI values, fouling was caused by NOM in two steps: a fast fouling process followed by a slow one. FI of the fast stage (FIF) was much greater than that of the slow one (FIS), showing the initial interaction would play a significant role in the fouling process over a short time. The results of mass balance suggested that a small fraction of DOC was responsible for membrane fouling caused by adsorption. Furthermore, both hydrophobic interaction and electrostatic interaction between NOM and the membrane determined the fouling behavior. Only a portion of foulants was removed after hydraulic washing. The sequence of NOM causing irreversible fouling was BSA > HA > SA, and the sequence of irreversible fouling after alkaline cleaning was SA > BSA > HA. The variations of FI values were consistent with the results of mass balances and flux recovery, which confirmed the FI was a simple and effective tool to describe the membrane fouling process.