The present study investigated the mechanism of surface vortex formation and free surface deformation in an unbaffled stirred vessel with on-axis and eccentric impellers having four paddle blades. To shed light on the mechanism of surface vortex formation and free surface deformation, we conducted experiments and numerical simulation in an unbaffled cylindrical stirred vessel. The surface vortex was found to be small in the eccentrically stirred vessel due to a vertical flow caused by the interaction between the discharged flow and sidewall. In the case of on-axis impeller, low-pressure zones at the rear faces of blades pulled the free surface, that causes the air entrainment. These phenomena could be seen in both the experimental observation and numerical simulation. The three types of transient free surface deformation were observed: local vortex formation due to the low-pressure zone mentioned above, entire rotation of free-surface elevation due to the macro-instabilities, and local elevation and deflection due to trailing vortices.