Field tests were carried out over a two-year period to obtain the drag and inertia coefficients of a surface piercing vertical cylinder fixed in irregular waves using an ocean research platform. The platform was set 2km offshore for collection of ocean environmental data on wind, waves and current. The wave forces were measured with two 2-component dynamometers set at either end of the test cylinder: data on waves were recorded using a wave height meter array, and orbital velocity was also measured with a 3-component electromagnetic current meter to confirm the accuracy of estimated values of orbital velocity calculated from wave data and linear wave theory. The following was found. (a) the values of CD and CM obtained from least squares fit of the complete force time series of a random wave record were well ordered as a function of Keulegan-Carpenter number KC defined by significant orbital displacement and the diameter of the cylinder, but those determined by the least squares fit on a wave by-wave basis were widely scattered. (b) ocean wave force exerted on the cylinder was well represented by Morison's formula; the ratio of predicted wave force by this formula to the directly measured force was 90%, and the ratio was very stable throughout the experiments.