Abstarct: Outlet gates have attracted a great attention in dam engineering and related industry. In this regard, the tasks of many investigations are focused on providing the discussion and solutions of related problems for such structures. Increasing the velocity and pressure reduction are the common problems in high head conduits, which lead to cavitation formation. Cavitation is the main factor of sever damages in outflow tunnels downstream of gates; therefore, it is necessary to decrease or eliminate its destructive impacts on the structures. Previous investigations have shown that air entrainment into the flow reduces cavitations damages; thus, aeration just downstream of the gates by air vents has become a common solution for such problem. The main aim of this research is to study the effects of gate lip on aeration process and its distribution downstream of the gates. The effect of gate lip is examined experimentally, and then is supported by numerical simulation. A physical model was constructed at Water Research Institute of Iran and examined with gate openings of 50%, 70% and 90% with the gate lip angel of 22.5o, and water head of 2m. Then, a numerical model was provided and calibrated by experimental results. The model was used to simulate the lip angles of 45o and 60o degrees to check its impact on the aeration mechanism. The results showed the reasonable agreements between the numerical and experimental information. It is also concluded that, increasing the gate lip angle changes the flow characteristics and thus, the air density downstream of outlet gates.