Lipid productivity, calculated from the biomass production and lipid content, is a key characteristic of the algal species and the culture strategy for biodiesel production. Both biomass production and lipid content are based on the biomass dry weight (DW). Due to inconvenient operation of the exactly measured DW, some reports used the estimated biomass DW by the optical density measurements and their calibration curves. At first, the different de-salted methods for DW determination of marine microalgae Nannochloropsis oceanica, a candidate for biodiesel production, were tested. The DW of non-washed samples was at least 2.4 times higher than those washed by reverse osmosis (RO) water, 0.5 M ammonium formate or 0.5 M ammonium bicarbonate or EC method (algal centrifuged and then washed with RO water till its electric conductivity became zero). The DW of EC method was the significantly lowest (p < 0.05) and used in the following experiments. The independent calibration curves from OD682 and DW were obtained by linear regression (R2 = 0.990 ~ 0.998) from 4 different culture conditions. Compared the measured DW of three condensed algal (OD682 > 1) and their deduced DW, estimated from measured OD682 of their diluted solutions and the linear equations, a great difference was found with the errors from -20% to 48%. The errors that compared the measured DW of the original algal cultures was compared with the calculated DW from the measured DW of their diluted solutions multiplied by dilution factor were lower (-1.63 ~ 3.62%). The results indicated that the biomass concentration of N. oceanica might be determined by DW measurement of de-salted algal taht washed with RO water, and then used for its lipid content or productivity calculation. The OD682 may be used as a reference of the growth of algal culture.