Abstract: The unicellular red alga,Rhodella reticulata,can synthesize some bioactive substances such as phycobiliprotein and exopolysaccharides.In this paper,the growth and physiological of Rh.reticulata under different KNO₃ concentrations were studied.The parameters,such as growth rate,exopolysaccharide,phycocyanin,pigments,nitrate reductase and SOD,were used in the assays.The result showed that Rh.reticulata grew the best under 0.75 g/L nitrate condition compared with nitrogen free medium,the alga also produced the highest activity of nitrate reductase,phycocyanin content（24 mg/L）,and caroteneid（1.42 mg/L）.While at the concentration 7.5 g/L nitrate,Rh.reticulata produced the most content of chlorophyll a（1.91 mg/L）.Higher nitrogen promoted the accumulation of Rh.reticulata exopolysaccharides.We got 1.633 g/L of exopolysaccharides when the concentration of nitrate was 30 g/L.With the increasing of KNO₃ concentration,the activity of SOD enhanced.