In this paper,the effects of five kinds of nitrogen source including NaNO_3,NH_4Cl,NaNO_3 and NH_4Cl,NH_4NO_3 and NH₂CONH₂ on the growth,fatty acid composition and EPA content of P.pyrenoidosus CCMP 2078 was studied.The results showed that the nitrogen source had significant influence on the growth of P.pyrenoidosus CCMP 2078.The highest cell density 8×10~6 cells/mL was obtained when cultured with NH₂CONH2,while the cell density in the mixture of NaNO_3 and NH_4Cl was higher than that in NH_4NO_3.With very low cell density,the growth was very slow either in NaNO_3 or NH_4Cl.The fatty acid composition was different under different nitrogen source conditions.However,the majority fatty acid were C14:0,C16:0 and EPA(C20:5).The highest PUFAs content was 24.27%when used NaNO_3 as nitrogen source,while the highest EPA content obtained was 12.93%when used NH_4NO_3 as nitrogen source.