Fouling characteristics of membranes with various molecular weight cut-offs (MWCO) were investigated. The effects of the molecular weights (MW) of humic acids and pre-treatment with PAC on membrane fouling were studied. It was found that the hydraulic resistance caused by fouling materials calculated using cake resistance in series model is a better indicator than the percentage of flux decline to assess the fouling of membranes with various MWCO. The effects of MWCO of membranes and MW of humic acids on membrane fouling can be explained by the different types of fouling mechanisms.