Field experiments were conducted at Hitokura Dam, Hyogo Prefecture, Japan, to improve the oxygen dissolution capacity of a conventional hypolimnetic aerator. Prior to conducting the experiment, dissolved oxygen(DO) level was examined from the bottom to the water surface near the aerator from spring to autumn. The average trend for the recent 5 years was that the DO level near the bottom rapidly declined from July to August, and only once several years, it took the bottom value far less than 2mg/L. In the main field experiment, the oxygen dissolution capacity of the aerator was measured when natural air plus pure oxygen was supplied by a compressor to the aerator’s intake port. The oxygen concentration of the mixed air was varied at the range up to approximately 70%O₂. The obtained results showed that the oxygen dissolution capacity of the aerator increased with the oxygen concentration of intake air, and dissolution ratio to the supplied oxygen reached 50% in the maximum case. Thus, we considered that the hypolimnetic DO level at Hitokura Dam can be improved, if supplying high oxygen concentration air, though a strong thermocline forms during midsummer.