Biofuel now is the spotlight of the world due to its ability to replace the fossil fuel without causing environment pollutions; therefore, many kinds of vegetable and microalgae have been researched for the production of biofuel. Known as one of the most potential microalgae for producing biodiesel, Nannochlopsissp. contains many useful fatty acids and about79% of triacylglycerol. However, because  Nannochlopsis sp. received from the stock of microalgaeof University of Technology, Ho chi Minh city was isolted from freshwater, its neutral lipids were very low. Thus, in order to determine the suitable conditions to increase the lipid content of Nannochloropsis species, after the biomass increase step, two experiments on citric acid supplementation (with 5 various concentrations: 0mM, 0.3mM, 0.6mM, 0.9mM and 1.2mM) and nitrogen - phosphorus deficiency (with 5 various proportions: 0.294:0.172; 0.15:0.075; 0:0; 0.294:0; 0: 0.268) were carried out. In this study, the Soxhlet system with n-hexane as the solvent was used to extract the neutral lipid of the microalgae grown in treatments of two experiments. The results from the experiment 1 showed that the treatment with 0.9mM citric acid got the higjhest biomass (1.053g/L), lipid percentage (17.7%) and lipid yield (0.186g/ dry weight). In the experiment 2, the highest values of the biomass, lipid percentage and lipid yield in Nannochloropsis sp. were recorded (0.644g/L, 24.748%, 0.159g/dry weight, respectively) when Nannochlocropsis sp. was grown in nitrogen and phosphorus free medium (N:P=0:0). The main factor induced the increase of lipid percentage and biomass was nitrogen-deficiency rather than phosphorus-deficiency.