Abstract: Background and Objective: The microalaga Thalassiosira pseudonana is widely used in aquaculture sector. The harvesting of microalgae biomass from culture medium is the big challenge in terms of technology and economy. The aim of the present study was to evaluate potential use of chitosan prepared from white leg shrimp shells to harvest the biomass of T. pseudonana and determine the contents of some biological compounds of the harvested biomass. Methodology: Effects of different harvesting conditions (pH, chitosan concentration and flocculation time) on the harvesting efficiencies of the T. pseudonana were evaluated. The recovery efficiency of microalgae biomass of chitosan and contents of some biological compounds of the harvested biomass were compared with some metal salts and centrifugation method. The effect of storage temperature on the contents of some biological compounds as well antioxidant properties of the harvested biomass was also examined. Results: The suitable conditions for harvesting the biomass were as follows: A pH of 6, chitosan concentration of 4 mg L–1 and flocculation time of 10 min. The flocculation efficiency of chitosan was much higher than that of some inorganic compounds including iron chloride (FeCl3), aluminum sulfate (Al2(SO4)3) and polyaluminium chloride (PAC). Compared with the centrifugation method, total contents of chlorophyll-a, chlorophyll-b, carotenoid and polyphenol of the microalgal biomass harvested by chitosan was significantly higher. Contents of biological compounds of the biomass significantly decreased after 2 week storage at different temperatures, the lower the storage temperature the higher the stable contents and antioxidant activity. Conclusion: Using chitosan as a flocculating agent could be the potential method to harvest the biomass of microalga T. pseudonana.