The propulsive performance and vortex shedding of oscillating foil, which mimics biological locomotion, were numerically investigated. The objectives of this study were to deal with unsteady force, in particular thrust force, exerted on the foil in pitching and plunging motions, and to explore the relation of the propulsive performance with vortex structures near the foil and vortex shedding in the near wake. The two-dimensional incompressible Navier-Stokes equations in the vorticity and stream-function formulation were solved by fourth-order essentially compact finite difference schemes for the space derivatives and a fourth-order Runge-Kutta scheme for the time advancement. To reveal the mechanism of the propulsive performance, the unsteady force and the shedding of the trailing- and leading-edge vortices of the foil were analyzed. The effects of some typical factors, such as the frequency and amplitude of the oscillation, the phase difference between the pitching and plunging motions, and the thickness ratio of the foil, on the vortex shedding and unsteady force were discussed.