An experimental study on the effects of water quality improvement using oxygenic micro bubbles aeration to deep layer in a reservoir was carried out for two systems using a micro bubble ejector. The first system sets the ejector near the benthic region of the reservoir, while the second one pumps water from the benthic region, inject micro bubbles near the water surface, and sends micro bubbles with water slowly in a larger pipe to the benthic region. As a result, improvement of dissolved oxygen concentration was recognized for both systems for the region between the injection level and the thermocline without the destruction of it. Moreover, from the view points of the electric conductivity and the oxidation reduction potential, water quality improvement was also confirmed. By comparison of the two systems, the second one got the better result for the improvement of dissolved oxygen concentration.