Abstract: The effect of nitrate- nitrogen on cells growth of Nannochloropsis oculata C170 and the synthesis of cellular compositions was studied.The impact was investigated in terms of the growth, nitrogen depletion and lipid content under different sodium nitrate concentrations, the variations of lipid accumulation, total sugar and protein etc, cellular compositions were also analyzed.The algae grew best when the nitrogen concentration was 42.5mg /L, in which its biomass ( 0.59 g /L) , specific growth rate ( 0.275d- 1) and productivity ( 50.98mg/ ( L·d) ) were the highest.The lipid content and nitrogen concentration presented a obvious negative correlation, and the lipid content ( 29.17%) and productivity ( 11.40mg/ ( L·d) ) peaked at 12.5mg/L. The polyunsaturated fatty acid increased significantly with the increasing of nitrogen concentration and the content of EPA increased from 8.06% to 18.76%.Under nitrogen-limiting conditions, the lipid content increased to 23.07%, and the ratios of the highest to the lowest value was 2.8. Meanwhile, the content of total sugar increased by about 76%, and the content of chlorophyll a, protein decreased by about 29% and 24%, respectively. The algae cells grew very well and reached a higher content of EPA when cultivated at a high concentration of nitrate- nitrogen. With a low concentration of nitratenitrogen, the accumulation of lipid was induced which was associated with the synthesis of saccharide, and also coincided with the degradations of protein and pigment.