Rate processes involved in the changes in the ratio of particulate organic carbon and particulate organic nitrogen (POC/PON ratio) in the decomposition of phytoplankton were clarified. First, we did two decomposition experiments using substrates whose high C/N ratio was more than two times higher than the Redfield ratio: (a) decomposition of Porphyridium purpureum (POC/PON=17) and (b) decomposition of Skeletonema costatum (POC/PON=6.5) in the presence of three times as much organic matter of glucose, which consequently increased the initial C/N ratio by more than a factor of four. In both experiments, the same mechanism was observed: POC decreased linearly while PON remained constant when the C/N ratio was high (>6). Then, we did decomposition experiments of extracted DOM of Skeletonema costatum in the presence and absence of a predator (heterotrophic nanoflagellates (HNF) and zooplankton). Finally, we developed a simple kinetic model of the phytoplankton decomposition process (PDP model). This model with fixed decomposition rate constants fits well with the results of the decomposition experiments where the initial C/N ratio and the volume ratio of POM and DOM differed by a factor of more than three. Our simple PDP model can explain well both the formation processes of semi-refractory POM (SR-POM) and the processes involved in the changes in the POC/PON ratio.