Experiments were made with either uniform spherical particles or non-uniform spherical particles using air and water as the working fluids. In the experiments, the diameter of the upriser, the diameters of the particles and the submergence ratio were changed systematically. The obtained data showed that the operation performance of an air-lift pump can be clearly characterized by the triangular relationship between the flow rates of the water and particles discharged and the how rate of the air supplied. A model which describes the hows in the upriser is proposed based on a momentum equation. The experimental and predicted characteristics of the flow are shown to be in good agreement.