Here, we tested temperature-size rule and resource-size rule in determining phytoplankton normalized biomass size spectral (NBSS) slopes in the Kuroshio region east of Taiwan. We used total biovolumes of phytoplankton as a proxy for resource supply instead of more commonly used inorganic nutrients, because inorganic nutrients cannot be a reliable proxy for resource supply in oligotrophic oceans. We find a positive relationship between phytoplankton NBSS slopes and total biovolumes, generally supporting the resource-size rule. Nevertheless, we found prevalence of small cells in spring, during which nutrient pulses are expected to benefit the large cells. This discrepancy can be explained by the none-steady state theory, where in the initial stage of nutrient enrichments in oligotrophic in spring, that small cells uptake nutrients faster than the larger ones. In general, size structures in the Kuroshio can be explained by the resource-size rule; nevertheless, the none-equilibrium condition needs to be bear in mind.