Deciphering how environmental factors determine phytoplankton size structure (using normalized biovolume size spectra, NBSS) has long been a research interest. The on-going debate has centered around two classic hypotheses: First, temperature-size relationship shows that phytoplankton NBSS slopes become steeper with increasing temperature. Second, resource-size relationship shows that phytoplankton NBSS slopes become shallower with increasing resource availability. To test the hypotheses, we collected 72 sets of pico- and micro-phytoplankton assemblages to examine NBSS from 9 cruises across 4 seasons from 2012 to 2014. We found that NBSS slopes did not show significant relationship with temperature variation, which is inconsistent with the temperature-size rule. By contrast, we found a positive relationship between NBSS slopes versus resource availability, generally supporting the resource-size relationship. The only exception occurred in winter and early spring, during which small cells were dominant as the resource pulse occurred and the NBSS slopes deviated downward from the expected positive resource-size relationship. This discrepancy may be explained as the early stage of resource supply; in the initial stage of resource enrichments in oligotrophic oceans, small cells with a high surface-area-to-volume ratio uptake resource more efficiently than the larger cells. After spring, the NBSS slopes basically follow the resource-size relationship.