Abstract: An experimental study is made on the flow characteristics of two-phase mixture of air and water flowing through the annular passage with three different clearances at normal temperature and pressure.It is pointed out that the two phase pressure drop is related to the geometric parameters of the annular narrow passage.A two-phase pressure drop correlation,which takes the geometric parameters of the annular narrow passage into account,is propsed by applying the separated flow model.The predicted values of the correlation agreement with the experimental data.