The effect of fresh thalli, dry powder and ethanol extracts from macroalga, Sargassum fusiforme, on the growth of a bloomforming microalga, Cylindrotheca closterium, was studied in coculture under controlled laboratory conditions. The results showed that the fresh thalli, dry powder and ethanol extracts exhibited strong inhibitory effect on the growth of C.closterium, and a concentrationeffect relationship was observed between the initial concentration of dry powder and the ethanol extracts and the growth inhibition of C.closterium. The relationship between the growth inhibition and the initial concentration of dry powder is y = -0.3801x² + 1.1636x, and it is y = -0.1445x² + 1.4279x between the growth inhibition and the initial concentration of ethanol extracts at 72 h. All the cells of C.closterium was dead in 3d when the concentration of dry powder reached 1.6 g/L, or in 2 d with the concentration of ethanol extracts reached 0.8 g/L. The latter exhibited stronger inhibition effects on C.closterium than the former because the concentration of ethanol extracts was expressed by the quality of dry powder before it was extracted. Under 1.6 g/L of ethanol extracts, the nuclei of C.closterium were broken in several hours. Up to 24 h, the cell membrane became obscure and difficult to be recognized, the whole cell shrank seriously and broke into fragments gradually.