Abstract: Phaeocystis globosa, which causes harmful algae bloom (HAB), was isolated from the South China Sea and grown in modified F/2 medium under varying nitrate concentrations. Cell growth and nitrate reductase (NR) activity were investigated. P. globosa grown without nitrate showed low-level NR activity, inhibited algal cell growth and no growth curve. The optimal nitrate concentration for algal cell growth and NR activity was 3.62 mmol.L-1. Under 4 different levels of nitrate, NR activity in cells peaked at day 9, but cell density peaked at day 16. Cell density and NR activity differed significantly nitrate concentration. Thus, eutrophic sea water with a high nitrate concentration is suitable for the growth of P. globosa and has great potential for developing into red tide.