In the test period of the Weir constructed in the estuary of Nagara River in 1994, the block of salt water in the upstream region of the Weir and the drop of DO in the bottom layer were observed. The necessity to research for these problems was pointed out. Investigation on the open channel flow was mainly conducting on its physical feature until now. The study on the quality of water was disregarding. Especially, our knowledge on DO in open channel flow is poor. Therefore, the object of this investigation is to examine the mixing of DO in the flow with the abrupt drop experimentally. The used experimental flume is made by acryl resin with the length 6 m and the width 15 cm. Increasing the accuracy of the measurement of DO, the experiments are conducted to use the water which make the DO decrease to about 50% DO of the water in the ordinary temperature by using the Sodium Sulfite. It is cleared from the experimental results that the amount of DO was closely related with the difference of water levels at before and behind of the abrupt drop.