Hydrodynamic forces on submerged rectangular cylinders oscillating relatively near a free surface were measured. Added mass coefficients and radiation waves measured simultaneously were in good agreement with the linear theory. When cylinders became close to a free surface, drag coefficients obtained from viscous damping component strongly depended on frequency unlike the case in infinite waters. The analysis of higher order terms of Fourier components of forces showed that there existed some mechanisms which were not explained by Morison type formula.
