Aeration by free surface vortex in hydro-electric facilities has been investigated for many years. The studies on free surface vortex have been mainly conducted by model experiment that required many works and costs. Combination of image velocimetry and numerical simulation is a powerful tool for estimation of occurrence of aeration by free surface vortex. The knowledge of aeration by free surface vortex is necessary to designing the hydro-electric facilities quickly and efficiently. In this paper, the technique to measure the velocity field and free surface profile simultaneously has been developed by using image processing techniques. The developed technique is applied to experiments on the aeration by free surface vortex in order to clarify relationship between free surface profile and circulation of flow fields. The results show some interesting phenomena related to aeration by free surface vortex.