A theoretical and experimental study is performed to determine the characteristics of the drag coefficient for a vertical circular cylinder in waves. Theoretically, the laminar boundary layer solution on the cylinder is obtained by applying the perturbation method to the Navier-Stokes equation. By using this solution, the separation point is determined and the point vortices to model the wake are shed from the separation point. The drag force is calculated by applying the momentum equation to the wave field including the shedding vortices. In the case of non-separation flow, the momentum defect in the boundary layer causes a drag force which greatly exceeds the surface friction force around a cylinder. The experimental values of the drag coefficient versus the Keulegan-Carpenters' number are compared with the theoretical. They agree quite well, which verifies the presented theory.