Wave forces acting on a test cylinder fixed to an offshore platform were measured to determine the nature of the hydrodynamic force exerted on a structure in random waves. Forces were measured by two dynamometers, wave height and direction were determined using a wave height meter array, and information on the orbital velocity was collected by a three-component current meter set near the test cylinder to confirm the accuracy of the phase and amplitude of this velocity calculated from the height meter array data. The inertia coefficient ( C _M) and the drag coefficient ( C _D) of the test cylinder in irregular waves were obtained, and laboratory studies developed a method of data analysis. The following was found : (a) Constant values of C _D and C _M obtained from least square fit of the complete force time series are well ordered as a function of significant value of K _C numbers while the values determined from least squares fit on a wave-by-wave basis are widely scattered with no apparent meaningful tendency. (b) Ocean wave forces exerted on the cylinder are represented well by Morison's formula ; the difference between the measured and predicted force by the formula is 10% and was very stable throughout the field experiments. (c) Transverse forces acting on the cylinder when it is fixed in irregular waves are weaker than those impacting when the cylinder is fixed in regular waves.