Abstract: The cultivation conditions of Phaeod actylum tricornutum , one of ocean microalgae, has been i nvestigated in this paper. It indicates that biomass concentration was 441 mg/L in ten days cultivation period under the following photoautotrophic cultivation conditions: a culture at 20 ℃, the o riginal pH 8.0, No.4 medium, urea as nitrogen source, nitrogen concentration 0.0 1 mol/L. The results show that photoautotrophic growth gives the lower biomass concentration than photoheterotrophy on sugars and P. tricornutum is a good source of EPA. This paper provides a good foundation for the further study of EPA poten tial production of P. tricornutum.