Abstract: Experiments were performed in a horizontal pipe for oil-water flow and oil-water-air flow.The phenomenon of phase inversion and its influence on hydrodynamic characteristics was investigated.The ratio of length to diameter of the pipe is 1250.A general model for describing density wave instability was put forward considering the Reynolds stress of dispersed phase based on two-fluid model.The results predicted by the model showed that phase inversion occurs when the volume fraction of dispersed phase is about 0.3,which agreed well with empirical data.The mechanism of phase inversion was explained by the theory of density wave instability.