Abstract: Heterotrophic dinoflagellates serve as an intermediate bridge connecting zooplankton and phytoplankton, and play a key role in material recycling and energy flowing in marine ecosystems.In this paper, fatty acid of heterotrophic dinoflagellate Oxyrrhis marina was analyzed after feeding on four phytoplankton, including Dunaliella tertiolecta, Heterosigma akashiwo, Phaeocystis globosa, and Navicula salinarum.Research shows that O.marina can feed on four kinds of phytoplankton, but ingestion rate on D.tertiolecta was higher than those on other three species.DHA (docosahexaenoic acid) was not observed in all prey, but found in the O.marina fed on them with the content up to (86.98±7.44) μg/mg C.These results indicate that O.marina can synthesize DHA independently, which means that O.marina had the ability to upgrade the nutritional value of phytoplankton, thereby efficiently delivery docosahexaenoic acid to the next trophic level.