The fluid propulsion mechanism of two oscillating flat plates is studied numerically using a discrete vortex method. Presently, the flat plates are assumed to be rigid. To analyze the closely coupled aerodynamic interference between the plates, a core addition scheme and a vortex core model are combined. The calculated wake pattern for a flat plate in heaving oscillation motion is compared using flow visualization. The effect of the wake shapes on the aerodynamic characteristics of the flat plate in pitching oscillation is investigated. The velocity contours at a certain distance behind the flat plates in pitching oscillations with several frequencies are plotted to investigate the possible thrust generation mechanism. The aerodynamic characteristics of the flat plates in pitching oscillation are also investigated. Nomenclature A ij = normal component of velocity induced at control point i by unit circulation at node j c = chord length, m cd = drag coefficient, D/0.5ρU 2 ∞c ct = thrust coefficient, T/0.5ρU 2 ∞c D = drag force, N d = distance between leading edges of plates, m h = heaving oscillation amplitude, m k = reduced frequency, ωc/U∞, rad N = number of nodes on flat plate ni = unit normal vector at control point i p = pitching oscillation amplitude, rad Qi = normal velocity induced at control point i by unit circulation at trailing edge rc = vortex core radius, m T = thrust force, N t = time, s U∞ = freestream velocity component in x direction, m/s X, Y = ground-fixed coordinates x, y = body-fixed coordinates α = angle of attack, deg or rad � = circulation of point vortex � l = panel length, m � t = time step, s θ = pitch angle, rad λ = wake wavelength, m ρ = freestream density, kg/m 3 � = velocity potential ωh = heaving oscillation frequency, rad/s ω p = pitching oscillation frequency, rad/s