Free surface vortex formation phenomenon at intakes is one of the most important problems in the water withdrawal process. In the present study, the free surface vortex formation was experimentally investigated. Experiments were performed on a single intake with three common intake withdrawal directions (vertical, horizontal and with angle of 45°). One of the main objectives of present study was determination of the strength of vortices. The tangential velocity component of vortices was measured by an Acoustic Doppler Velocimeter (ADV). The results indicated that, by varying the withdrawal angle from vertical to horizontal, the strength of vortices decreased by about 31%. Based on analyzing of experimental data, an empirical relationship between the circulation number and Froude number for various submergence depths was developed for each withdrawal direction. The critical submergence for an air entraining vortex at intakes was also investigated. The results indicated that the critical submergence was considerably affected by the changing of withdrawal direction. It could be concluded that the minimum of critical submergence was occurred at horizontal direction. Based on analysis of the experimental data for each withdrawal direction, an empirical equation was also obtained, which is used to calculate the critical submergence. The results also were compared and analyzed by other researcher’s investigations and showed satisfactory agreement.