The present disclosure relates to strains of Thraustochytrium genus, including a high content of polyunsaturated fatty acids, and a method of producing a biomass using the same. According to the novel CJM01 microalgae of Thraustochytrium genus of the present disclosure, the content of lipids in the biomass and the content of unsaturated fatty acid such as docosahexaenoic acid in the biomass are high, so that the microalgae itself, a biomass produced by the culturing and fermentation of microalgae, a condensate of the biomass, and a dried product of the biomass are very useful as a feed composition.