Two-dimensional (2D) numerical simulations of multiphase flows past a circular cylinder close to free surface waves were performed to investigate an interaction of vortex formation around a cylinder with periodic waves by utilizing waves2Foam. The lock-on responses of lift coefficient at low frequencies were lower than those of the natural response due to the existence of “out-of-phase” between the fluctuations of lift coefficient by vortex formation and the vertical force fluctuations by wave motions. The resonant effect of an external excitation by the periodic waves on the lift coefficient fluctuations was not significant despite the occurrence of lock-on.