The viscous flow field around two-dimensional flapping (heaving and pitching) foils was numerically computed. The structural characteristics of caudal vortices were investigated and the contour curves at different phase angles were obtained. The relationships between the structural characteristics of the vortices and the force acting on the foil and between the widths of the caudal vortex street and of the caudal flow field were analyzed. A method to determine the shedding frequency of the vortices was proposed. The viscous flow field around two-dimensional flapping (heaving and pitching) foils was numerically computed. The structural characteristics of caudal vortices were investigated and the contour curves at different phase angles were obtained. The relationships between the structural characteristics of the vortices and the force act on the foil and between the widths of the caudal vortex street and of the caudal flow field were analyzed. A method to determine the shedding frequency of the vortices was proposed.