Abstract: In order to investigate the effects of nitrogen sources and the concentrations on fatty acid composition in microalgae,a marine diatom,Phaeodactylum tricornutum,was cultured in batch with varied nitrogen sources(NO3-,NH4+,NH2CONH2)and at different concentrations.Its growth and fatty acid composition were investigated.The results showed that P.tricornutum grew poorly in the cultures without addition of any nitrogen source,however,polyunsaturated fatty acids(PUFAs)and C18 fatty acids(C18∶0,C18∶2(n-6),C18∶3(n-6))accounted for higher percentages of total fatty acids compared to those with addition of the nitrogen sources.At low concentrations of the nitrogen sources(4+,and grew faster with NH2CONH2 at high concentrations (>3.5mmol/L).With NH4+ or NH2CONH2 as nitrogen sources,EPA(eicosapentaenoic acid)and PUFAs in percentages of the total fatty acids increased when nitrogen concentrations were raised from 0.9 to 7.0mmol/L.With NO3-,the EPA and PUFAs percentages increased to the maximum at 1.8mmol/L NO3-,then decreased with the increased NO3- levels,while the highest EPA Was 16.7% of the total fatty acids.EPA content on a dry mass base(w/w)increased with the increasing nitrogen concentrations within a range of 0.9 to 7.0mmol/L regardless of the nitrogen sources.At 7.0mmol/L NH2CONH2,the alga achieved the highest contents(2.6%EPA and 4.4%PUFAs of the dry weight,respectively).PUFAs content on dry mass decreased with increased.NO3-,increased with enhanced NH2CONH2,but showed small variation with added NH4+.