Well-dispersed organic nanoparticles of porphyrin with narrow size distribution were successfully prepared in aqueous solution based on the "ion-association" method. No self-aggregation of the constituent porphyrin chromophores was confirmed. The nanoparticles exhibit interesting optical properties; a large bathochromic shift in the absorption and a remarkable resolution increase in the fluorescence compared to those for the corresponding aqueous-phase porphyrin.