Abstarct: In the present thesis, the aeration downstream of gates is studied by evaluating the performance of aerators in bottom outlets of dams. This evaluation is baxsed on the rate of aeration. Experimental data from hydraulic models of outlets built in Water Research Center laboratory were collected and combined with data from other references. According to previous studies and considering the performance of gates, data analysis is classified into two main sections, downstream of one gate and between two gates. According to new studies on the collected data and the results of pervious researches, the first section is classified into two sub-sections, with or without cross-sectional expansion of downstream tunnel. It is because there are two different aeration mechanisms in this section. The collected data were insufficient for analysis and also there were no data for aeration between two gates while the emergency gate is operating. To complete the data set, new experiments were performed on the existing models of Water Research Center laboratory. Study of the laboratory experiments showed that the results from the equations proposed by other researchers had lots of differences with the experimental results and the evaluation of the performance of aerators would have considerable errors by using these equations. Therefore the effective parameters on aeration downstream of gates are determined and by paying special attention to geometry and hydraulic characteristics of water flow, new equations for each section are presented. Finally by studing air-water flow in hydraulic structure,rate of aeration in bottom outlet gate determine using various parameters of the flow.