Abstract: As the main consumers of phytoplankton, heterotrophic dinoflagellates play key roles in carbon cycling and nutrient regeneration in marine ecosystem. Oxyrrhis marina was a model predator in laboratory-based feeding studies. To assess the effects of food type and particles size on grazing rate, growth rate and cell volume of heterotrophic dinoflagellate, O. marina were provided with either individual or combined prey. The results indicated that the O. marina has ability to consume 7 taxa, belonging to Chromalveolata, Haptophyta, Chlorophyta, Bacillariophyta and Ochrophyta, respectively. The feeding activity of O. marina strongly depends on the prey size, which influenced significantly the grazing rates, growth rates as well as cell volume. O. marina was ability to fed prey with size ranged from 2 μm to 128 μm. Prey with diameters ranged from 7.4 to 13 μm, e. g. Dunaliella tertiolecta, Heterosigma akashiwo and Thalassiosira weissflogii, resulted in the highest ingestion rates, growth rates and cell volume in O. marina. The Ingestion rate of O. marina on Aureococcus anophagefferens, Emiliania huxleyi, unicellular Phaeocystis globosa which diameters ranged from 2 to 4.5 μm, and D. brightwellii with diameters up to 128 μm was decreased significantly. Furthermore, D. brightwellii was not ingested by O. marina once alternative prey was available. Heterotrophic dinoflagellates could play a significant role in regulation of size spectrum and population dynamics of phytoplankton in marine ecosystem due to their size-selective feeding.