The microalga Nannochloropsis sp. was cultured under different initial population densities (IPDs) ranging from 0.11 to 9.09 g L⁻¹. The IPD affected the biomass and lipid accumulation significantly. The algal cultured with higher IPD resulted higher biomass concentration (up to 13.07 g L⁻¹) in 10 days growth. The biomass productivity with 0.98 g L⁻¹ IPD was 0.75 g L⁻¹ d⁻¹ which was higher than that of other IPDs. For IPDs ranging from 0.11 to 0.98 g L⁻¹, with the increase of IPD, the biomass productivity increased, while for IPD over 0.98 g L⁻¹, the biomass productivity decreased. Lipid content of the algal culture started with 0.11 g L⁻¹ IPD reached to 42 % of dry weight. But with the increase of IPD, the lipid content decreased. Lipid composition was analyzed using thin layer chromatography coupled with flame ionization detection (TLC/FID). Seven lipid classes were identified and quantified. The main reserve lipid, triacylglyceride (TAG), accumulated under all different IPD conditions. However, with the increasing IPD values, TAG content decreased from 59.1 to 23.5 % of the total lipids. Based on these results, to obtain the maximal biomass productivity and lipid productivity of Nannochloropsis sp. in mass cultivation systems, it is necessary to select an appropriate IPD.