This study was performed to determine optimum conditions of mass cultivation of chlorella sp. FC-21 cultivated under light emitting diode (LEDs). Specific growth rate and cell concentration were measured for the reactors at the illuminations of different wavelengths of LEDs. Among the culturing temperatures, 220C was the optimum temperature for the growth of Chlorella sp. FC-21. Among various types of LEDs, Red LED were the most effective light source, and also greatest increases of specific growth rate and cell concentrations were obtained when light intensity of red LEDs increased. The specific growth rate decreased when initial cell concentration increased due to the shading effect of each cell in the reactor. Comparison with common light source, fluorescent light, showed less effective than that with Red LED. To derermine beneficial effect of aeration to cell cultivation, The aeration rates varied from 0 to 2.4 vvm under the illumination of 4,400 lux of red light emitting diode. The highest specific growth rate of 1.51 day-1 was obtained at the aeration of 0.7 vvm and lower specific growth rates were obtained for other aeration tests. Furthermore, the highest biomass concentration (1.02 g/L) was also obtained at the aeration of 0.7 vvm. Therefore, aeration of 0.7 vvm was determined to be the optimum aeration rate for the cultivation of Chlorella sp. under red light emitting diode. Semi-continuous cultivation was conducted using red LED because red LED was found to be the best light source for chlorella sp. FC-21. During cultivation, phosphate and nitrogen were quickly diminished where cell concentration of chlorella was inversely proportional to the concentrations of phosphate and nitrogen in culture solution. To increase the period of dilution of culture solution, additional amounts of phosphate and nitrogen were inserted in the culture solution to increase the concentrations of phosphate and nitrogen. The cell concentrations of chlorella increased in the modified culture, but cell diameter was diminished as the dilution of culture was periodically conducted. When considered the cell concentration and cell diameter during the cultivation, amount of biomass produced was maintained constant. The other purpose of this study, treatment of contaminated lake water and harvesting of microalgae biomass using the proposed process which is consisted of micro air bubble, flocculation and flotation. Flocculant toxicity should not be ignored but organic polymeric flocculant which is biodegradable and less toxic offer and alternative and environmental friendly way to aggregate suspended particles. This micro air bubble, coagulation and flotation effectively controlled, for example removal efficiency of turbidity were more than 95 %. Micro air bubbles coagulation and flotation process could be effectively used as an alternative treatment method for the purpose of control of water quality and harvesting method for microalgae biomass.