Hydrodynamic forces acting on rough circular cylinders in a harmonically oscillating flow at high Reynolds numbers were experimentally investigated. Especially effects of the roughness height and roughness density were investigated. Two kinds of roughness height-5mm, 10mm and three kinds of roughness density-20, 60, 80% were used in experiments. Experiments were carried out using a large circular cylinder with 1.2m diameter and with 1.5m length at NKK's Tsu Ship Model Basin. The cylinder was fixed to the main carriage. The cylinder was forced to oscillate harmonically with the main carriage. In-line and lift forces acting on cylinders were measured at Keulegan-Carpenter numbers over 6 up to 50 and at two Reynolds numbers-0.5, 1.0×10⁶. Drag coefficients, added mass coefficients, maximum lift coefficients and periods of lift force are obtained. Experiments show that the roughness has a great effect on the hydrodynamic coefficients even at high Reynolds numbers. In the range of the value of roughness parameters used in these experiments, a roughness height has a more considerable effect on the hydrodynamic coefficients than a roughness density.