Microalgae blooms are one of the most serious environmental problems that greatly affect the living environment of aquatic organisms. They deplete the oxygen source in the water, seriously depleting the respiratory oxygen source for organisms living in the water body. Some species of microalgae are also capable of producing toxins, typically blue-green algae, poisoning aquatic organisms and causing death, health effects on human life when ingested. This paper presents the results of research on mapping algae concentration distribution to support water quality assessment for Dau Tieng reservoir. Landsat satellite imagery was used for qualitative and quantitative assessment of algae presence in the lake area. Calculation results showed that algae concentration had a linear correlation with the band ratio of the 3-band algorithm (3BDA (2), and was a combination of spectral reflective bands in the green wavelength bands GREEN, RED and near infrared NIR. We simulated the spatial distribution of algae concentrations on the entire lake area on 3 points of time when satellite images had been collected. Result of this study showed that remote sensing technology can be used for monitoring the appearance and change of algae concentration in water, which is an essential measurement for timely providing warnings as well as assisting in monitoring and managing water environment quality to minimize damages to people and the ecosystem.