In this work the gas dispersion of a Rushton disc turbine impeller was investigated in a horizontal non-baffled stirred vessel in comparison with the normal vertical baffled stirred vessel. The Rushton turbine and vertical baffled vessels are commonly used for gas dispersion mixing. The flooding/loading transition and the gas-liquid mass transfer coefficient k(L)a were measured at the gas flow rates of 0.5-2vvm. The results indicated that k(L)a against energy dissipation rate in the horizontal tank was better than that in the vertical baffled tank, and roughly equal loading/flooding transition for the two kinds of vessels was obtained. The correlations of k(L)a and the loading/flooding transition were correlated according to the present data.