A new type of semi-floating structure for navigation aids is under development, which is made of a vertical, buoyant cylinder connected to an anchor with a universal joint. The response of the cylinder to sea waves is studied by the theoretical analysis, hydraulic model tests, and field tests. The study has yielded the following conclusions:
　1) The oscillation of the cylinder in regular and irregular waves is quite accurately calculated by the theory which takes into account both the linear forces and nonlinear damping terms.
　2) The calculated amplitude of the cylinder oscillation generally increases as the estimated value of drag coefficient is decreased.
　3) The characteristics of the cylinder oscillation in irregular waves differ from those inregular waves.
　4) The numerical simulation is found very useful to solve the equation of motion involving nonlinear terms.
　The present analysis will be easily extended to a single point mooring system with a single vertical cylider.