The analysis of the wave forces acting on piles is very important for the design of offshore structures such as oil-drilling platforms. Since the analysis requires good informations on the orbital velocities of water particles and the drag coefficient of cylinders, large scale tests have been conducted in a wave channel of 105 m in length with test piles of 7.62 and 13.98 cm in diameter. The drag coefficient of a circular cylinder under the wave action was determined from the moments of wave forces with actual orbital velocities directly measured, showing almost the same value with the drag coefficient in steady state flow. The limiting wave height on a gently inclined bottom of 1 to 100, the elevations of wave crests above the mean water level, and the virtual mass coefficient of a circular cylinder were also investigated in the experiment.
　Based upon the results of the experiment and other investigations, a computation method of the wave forces on a vertical cylinder been prepared and presented in this report. The wave forces calculated with this method showed good agreements with experimental date of the present author and others. For a vertical pile of uniform section, several design diagrams are available for convenience of the wave force design.