Microalgae as phytoplankton are unicellular algae in microscopic size floating in the water, in which diatom is the biggest phylum in term of species numbers and biomass. Initial cell density is one of the factors closely related to algal biomass and time when algae reach the maximum biomass. Skeletonema costatum (Greville) Cleve grows the best in the Aquil medium with initial cell density 30,000 cells/ml, light intensity 3,000lux + or - 500, temperature 25°C + or - 2: the long-chained cells, the darker chromatophores, and the typical growth curve.