Abstract: Experiments were carried out to investigate the sonic velocity of air-powder two-phase flow in a horizontal constant cross section duct. Based on the forming principle of choking phenomenon, an approach is proposed to measure the sonic velocity of air-powder two-phase flow. The relationship between the mass flow rates of such an air-powder two-phase flow and inlet stagnant pressure is obtained with different solid-to-air ratios, thus determining the pressure under which the choking phenomenon takes place and, further, giving the relationship of the ratio of back pressure to critical pressure and the solid-to-air ratio. Taking the continuity equation of air-powder flow into consideration, the apparent sonic velocity of two-phase flow can be given. Furthermore, the apparent sonic velocity is compared with theoretical sonic velocity calculated on the theory of two-phase equilibrium flow.