Wave forces on a fixed, vertical circular cylinder calculated by using the Morrison equation and the wave diffraction theory are compared with each other, and the wave force calculation for the transition from drag to inertia dominated region is studied. To calculate the Morrison equation, the inertia and the drag coefficients obtained by Chakrabarti in the wave tank tests are used, and the depth integration of the Morrison equation is then used to obtain the wave forces acting on the structure. Wave forces on large circular cylinders are calculated by using MacCamy and Fuch's wave diffraction theory wherein viscous effects and wave non-linearities are neglected in the theory. Various forms of the Morrison equations are examined and compared to study the extent the simplifications made in the wave force calculation will effect. The drag force and the inertia force components in the Morrison equation are compared with each other to investigate their relative importance in the wave ranges. A scaling analysis of the Morrison equation is derived to show the order of magnitude is derived to show the order of magnitude of the drag and the inertia force components.