Abstract: To understand the characteristics of ventilated cavity,ventilated cavity flow around a hemisphere cylinder is experimentally studied by applying the high speed visualization technolo-gy.The results show that the gravity effect and air entrainment play a major role in forming mul-tiphase flows of ventilated cavity.Froude number and air entrainment coefficient are defined. Five multiphase flows of ventilated cavity around a hemisphere are denoted,which are transpar-ent bubble flow,transparent slug flow,transparent layering flow,air-water mixture flow and semitransparent air-water mixture flow.The influence of flow parameters on fluid structure is analyzed,and there are two stages.One is the stage where the gravity plays a critical role,and the other is the stage where the gravity effect is limited.In the stage where the gravity plays a critical role,when air entrainment coefficient is fixed,fractured cavity changes into steady cavity attaching to the surface of the cylinder as Froude number increases;when Froude number is fixed,the scale of fractured bubble increases as air entrainment coefficient increases.In the stage where gravity effect is limited,when air entrainment coefficient is fixed,Reynolds number and turbulence intensity increase and bubbly wakes created by ventilated cavity become turbulent as Froude number increases;when Froude number is fixed,cavity number based on cavity pressure decreases and cloud cavity around a cylinder changes into transparent cavity as air entrainment co-efficient increases.Finally unsteady bubble shedding is described when fluid structure is domina-ted by gravity and by re-entrant jet.