The evolution of wake structures and variation of the forces on a flat plate in harmonic oscillatory and in-line combined flows are obtained numerically by improved discrete vortex method. For the oscillatory oncoming flow cases, when Kc number varies from 2 to 40, the vortex pattern changes from a "harmonic wave" shaped （in a range of small Kc numbers） to a slight inclined "harmonic wave" shaped （in a range of moderate Kc numbers）, then to inclined vortex clusters with an angle of 50 ° to the oncoming flow direction （at Kc = 20）, at last, as Kc number becomes large, the vortex pattern is like a normal Karman vortex street. The well predicted drag and inertia force coefficients are obtained, which are more close to the results of Keulegan & Carpenter’s experiment as compared with previous vortex simulation by other au- thors. The existence of minimum point of inertia force coefficient Cm near Kc = 20 is also well predicted and this phenomenon can be interpreted according to the vortex structure. For steady-oscillatory in-line combined flow cases, the vortex modes behave like a vortex street, exhibit a "longitudinal wave" structure, and a vor- tex cluster shape corresponding to the ratios of Um to U0 which are of O (10-1), O（1）and O （10）, respectively. The effect on the prediction of forces on the flat plate from the disturbance component in a combined flow has been demon- strated qualitatively. In addition to this, the lock-in phenomenon of vortex shedding has been checked.
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