The present work was conducted to improve the gas-induction performance over a preliminary gas-inducing impeller designed in our previous work. For the first time, a series of liquid-inlet holes with different diameters were drilled separately so as to examine their effects on the gas-induction process and also to find the optimum size. The critical impeller speed (N-CG) and the overall gas hold-up (epsilon(G)) were measured experimentally in order to evaluate the properties of the gas-inducing impeller. A mathematical model, which takes into account the geometrical parameters of the gas-inducing impeller, was proposed to predict N-CG of the gas-inducing impeller with or without liquid-inlet holes. The predictions are in good agreement with the experimental results. The results also show that the optimum gas-induction properties would be obtained with respect to the lowest value of N-CG and the highest value of epsilon(G), when the diameter of the liquid-inlet hole is approximately 0.5 times the diameter of the gas-inducing pipe. (c) 2013 Elsevier B.V. All rights reserved.