Dynamics and optical nature of fluorescent dissolved organic matter (FDOM) were investigated in two rivers (Kurose and Ohta) in Hiroshima prefecture, Japan during 2002-2003, by measuring dissolved organic carbon (DOC) and three-dimensional excitation emission matrix fluorescence (3-D EEM). In monthly collected samples, DOC varied from 43 to 146 μM C at upstream sites in both rivers, and from 130 to 349 μM C and from 45 to 164 μM C in Kurose and Ohta downstream, respectively. The 3-D EEM of FDOM in the river waters identified three characteristic peaks, indicating the occurrence of fulvic acids (peak F), fluorescent whitening agents (peak W), and protein-like substances (peak T). The upstream FDOM in the both river waters and the downstream FDOM in the Ohta river contained the peaks F and T, identified by comparing with that of the Suwannee River Fulvic Acid (SRFA) and tryptophan and their photo-irradiated standards. The 3-D EEM of FDOM in Kurose downstream waters, however, contained the peaks W and T. The ratio of fluorescence intensity for peak F or W to DOC (FI/DOC-index) was estimated to be high (2.00 ± 0.51 to 2.09 ± 0.38 QSU (quinine sulphate unit)/μM C) at Kurose downstream compared to Ohta rivers (0.73 ± 0.35 to 0.74 ± 0.38 QSU/μM C) and to the Kurose River upstream (0.55 ± 0.22 to 0.65 ± 0.21 QSU/μM C), and the absolute FI values were several times higher in Kurose downstream waters than Kurose upstream and Ohta river waters. Moreover, the studies on the photo irradiation experiments of the standard FDOMs indicated that the peak W is easily photo-decomposed while the peak F appears to be photo-resistant. These results indicate that the chemical properties of FDOM in the Kurose downstream waters are different from those in the Kurose upstream and Ohta river waters.