Skeletonema costatum is the dominant phytoplankton in Changjiang Estuary. Blooming of Skeletonema costatum arose several times in Changjiang Estuary in recent years. In order to study the relationship between concentration of nitrate and the Skeletonema costatum blooming. We studied the growth of Skeletonema costatum in different concentration of nitrate. Growth rate, PH, dissolved organic carbon (DOC) and maximal uptake rates of nutrients were educed through the experiment. The results showed that low nitrate concentration limited the growth of Skeletonema costatum although the concentration of phosphate was high. The optimum N/P ratio was 15.7 among different N/P ratios tested in the experiment. The closer the N/P ratio was to the Redfield ratio 16:1, the higher was the growth rate of phytoplankton. The growth rate and concentration of cell, when N/P ratio was over 15.7 were much higher those when N/P ratio was under 15.7. Under condition of low nitrate concentration, the dependence of Skeletonema costatum growth on nitrate was increased. The release of DOC was promoted obviously by enrichment of nutrient.