Abstract: The unsteady characterics of cloud cavitating flow arounda hydrofoil are investigated by the experimental and numerical methods.Experiments on a hydrofoils are carried out in a rectangular test section ofa cavitation tunnel. A high-speed video camera is used to visualize theunsteady flow structures. The caculations are conducted on thetwo-dimensional hydrofoil section, based on a single-fluid model of thecavitation: the liquid/vapor mixture is considered as a homogeneous fluidwhose compsition is regulated by mass tranfer equation. The RNGk-\varepsilon turbulence model with modified eddy viscosity coefficient isused in the computation, and the coefficient is related to the vapor andliquid densities in cavitated regions for simulating the cavitating flow.The experimental and numerical results show the unsteady features of theperidical cloud cavity departure; the increasing of a local pressure is amain factor to induce ruture of a cloud cavity; an interaction between there-entrant jet and the cavity interface in the closure region can lead tothe increasing of the local pressure; the adverse pressure gradient ismainly responsible for the generation of the re-entrant jet.