We investigate numerically and experimentally the mechanisms of surface air entrainment in the vessels equipped with the long-short blades agitator. VOF method coupled with LES model is used to visualize the surface air entrainment process. In the case of partial submergence of the long blades (LBs), the interaction of the LBs with the liquid free surface creates a depression behind the LBs. Backfilling of the liquid into the depression leads to gas separation and entrapping into the liquid. The critical tip velocity of the LBs, u(tip,c), for the onset of gas entrainment is measured in vessels with diameters, T=200 approximate to 600 mm. It is found that when H/T1.0, u(tip,c) is determined by the LBs, independent of the liquid level. u(tip,c) is also affected by the size of the vessel through the diameter of the sweeping circle of the LBs, but for substantially large vessels, it approaches a constant value. (c) 2017 American Institute of Chemical Engineers AIChE J, 63: 316-325, 2018