Abstarct: Spillway of high-head dams are often subjected to cavitation damage caused by the hydrodynamic effects of very high water velocities. A number of structures around the world and in Iran have been destroyed or damaged due to the cavitation phenomenon. It is generally accepted that presence of 8 percent of the air inside the water flow can help to prevent cavitation or stop its risks. Aeration can be done by installing aerators in the flow path. Air entrainment into the water flow and behavior of air-water mixture flow are complex and difficult to analyze. Despite of many studies performed in connection with cavitation, there is still a lot of information needed for the design of the spillways. For example, there is not a reliable relationship or clear method to determine the distance between the aerators and little information about the air-water flow over the spillway is available. The aim of the present study is to investigate the aeration mechanism and the effective parameters on air-water laxyer growth over the aerator nappe. In this dissertation the air concentration distribution in the air-water lower laxyer of aerator nappe was studied and baxsed on that an empirical relationship for the prediction of air concentration was presented. The proposed equation for air concentration distribution was then compared with other researchers` experimental data. Comparison of this equation, with data of other researchers indicated a good agreement. The air-water laxyer growth in the nappe was also studied analytically and a relationship was established between the thickness of air-water laxyer and eddy diffusivity. Finally, the results of the present analytical studies were compared with analytical and experimental measurements of other researchers. The comparison showed that these results were approximately located in an acceptable range with the results of previous researchers.