Natural organic matter (NOM) from five sampling stations of a shallow, eutrophic lake, Lake Kasumigaura was analyzed in four different seasons of a year by several NOM characterization methods including GPC and pyrolysis GC/MS with the objective of elucidating its characteristics as well as studying its temporal and spatial changes. A comparison was also made among the characteristics of NOM from this and two other lakes. The study showed that NOM in Lake Kasumigaura had comparable DOC, UVA260, and THMFP with that of Lake Inbanuma, but much higher than that of Lake Tsukui. Both molecular weight and polydispersities remained similar among those three sources but these values were significantly smaller in comparison with Suwannee River NOM. The similarity index analysis based on pyrochromatograms was useful to evaluate spatial and seasonal changes in NOM characteristics. Distinctive characteristics of NOM at KS1 (station near the mouth of Sakura River) were better explained by the difference in pyrochromatograms rather than in general water quality. Except for KS1, the change in NOM characteristics in the lake water was found to be more pronounced in different seasons than at different locations.