The present paper deals with the nonlinear swaying motions of a moored cylinder in regular waves. With the consideration of the higher order forces caused by the interaction of the cylinder and waves, Mathieu equation is introduced as the equation of swaying motion. Therefore it is expected that an unstable swaying motion occurs when the ratio of the natural period of a moored system to wave period becomes certain values. The experiments were carried out to confirm the possibilities of the unstable swaying motions of the moored cylinder in regular waves, and the results obtained by the experiments show qualitatively good agreement with the theoretical prediction. But further investigations are necessary as to the quantitative estimation of stability limit in this phenomenon.