Objective: To study the anti-tumor activity and immunomodulating effects of polysaccharide from Gracilaria tenuistipitata in vitro. Method: The S-180 cell and spleen lymphocyte were cultured in vitro with different dosage of two polysaccharides (GTHSP and GTLSP) separated via ion-exchange and tested by MTT method. Result: High sulfate-containing polysaccharide GTHSP could markedly induce proliferation of spleen lymphocyte (P<0.01), while no such effect was found with low sulfate-containing polysaccharide GTLSP. Additionally, none of the two samples showed inhibition effect on the growth of S-180 cell. Conclusion: Neither of the tested polysaccharides had anti-tumor activity. It was also confirmed that the immunomodulaiting effect was related with the content of SO42- in sulfated polysaccharides and the polysaccharide GTHSP with high SO42- content showed immunomodulaiting effect.