The objective of this study was to effectively evaluate pilot-scale membrane process versus pilot-scale granular activated carbon (GAC) and full-scale coagulation processes with respect to natural organic matter (NOM) removal, by determining an effective pore size distribution (PSD) and an effective molecular weight cutoff (MWCO) of the above-mentioned three processes. Currently, drinking water treatment processes have been evaluated in terms of NOM removal only with respect to dissolved organic carbon (DOC) and ultraviolet absorption (UVA). However, it is difficult to determine which portion of NOM (in terms of molecular weight) is removed by a specific treatment process. Thus, effective PSD concept for NOM removal process was introduced and used to evaluate the efficiency of membrane process, as compared to GAC and coagulation processes. Effective PSD and MWCO of a membrane were determined from MW distributions of NOM included in feed and treated samples, using the fractional rejection method and a corresponding slope at a certain MW, respectively (Lee et al, 2002); from this method, even GAC and coagulation processes were revealed to have "effective PSD and MWCO", and can be effectively compared to membrane processes. MW distributions of feed and treated solutions were measured by high performance size exclusion chromatography (HPSEC) method with UV detector. For this study, pilot-scale nanofiltration/ultrafiltration membrane and GAC (a filter- adsorber type) plants were set up in a full-scale conventional water treatment plant in the City of Changwon, Korea. Includes 6 references, table, figures.