Flow fields around an oscillating airfoil at low Reynolds numbers are extremely unsteady. In particular, the pitching oscillation at the high angle of attack encourages vortex dynamics in the separated shear layer near wake of the airfoil and produces unsteady vortex motions. To evaluate the force correctly, it is necessary to know the unsteady properties determined from the vortex dynamics because that unsteadiness is known to make the force greater. The purpose of the present study is to investigate the relationship between unsteady fluid forces and vortex structures for a discoid airfoil during the pitching and heaving motions. Furthermore, the effect of phase difference θ between pitching and heaving motions on vortex dynamics is also observed. It is confirmed that the unsteady fluid forces were affected by the vortices shed from the airfoil edge during upstroke in pitching oscillation. There are two peaks in the fluid force during one pitch-oscillating due to the vortex behavior. The vortex behavior was strongly affected by the reduced frequency due to pitching oscillation, and the fluid force acting on the airfoil model increases with increasing the reduced frequency. During coupled heaving and pitching motions, phase difference between pitching and heaving motions affects the vortex behaviour in the wake of the airfoil.