Hydrodynamic forces acting on three dimensional circular cylinders in an oscillating flow are investigated experimentally. Forced surge tests were carried out on the finite length circular cylinders of which the ratios of the length to the diameter are varied from 1 to 10. The drag coefficient, the added mass coefficient and the lift coefficient were obtained. They were compared with those of two dimensional circular cylinders. The experiments showed that these coefficients of three dimensional circular cylinders are much smaller than those of two dimensional circular cylinder.