Hatchery is one of important factors to maximize shrimp production. One of supporting factors is the availability of qualified natural foods in sufficient quantities. The aim of this study was to know the addition effect of extract Sargassum to culture medium for population growth of Chaetoceros gracilis. Culture medium used for microalgae C. grasilis was a 12 liter jar with density 105 cell/mL. The Sargassum extract was obtained by processing Sargassum waste through anaerob fermentation, then was extracted, and considered as liquid organic fertilizer. Tested treatment was A = 0 ppm (without Sargassum extract), B = 10 ppm of Sargassum extract, C = 20 ppm of Sargassum extract and D =30 ppm of Sargassum extract for three replications. It was found that the population density of Chaetoceros increased and the highest algae density occurred in treatment D on day 5, followed with C, B and A, which were 353,433 cell/mL, 336,833 sel/mL, 308,867 cell/mL and 244,433 cell/mL. This illustrates that the addition of high doses has affected the population density of microalgae C.gracilis.