Abstract: The vortex structure shed from the trailing-dege of a constant-rate pitching airfoil was visualized using hydrogen bubble technique. Its time evolution, and the effects of pitching rate were studied. Combining the trailing-dege vortex structure with the (previously obtained) airfoil upper-surface separated vortex structure, a complete flow structure was obtained. The mechanism of dynamic stall was studied based on the flow structure and vorticity dynamics theory, and insights were provided.