A simple passive technique of vibration isolation for flexible structures by nonlinear boundaries is investigated, which to our best knowledge is the first study of its kind reported in the literature. The equations of the structure are derived with Hamilton’s principle. An iterative analytic method is investigated to improve the accuracy of the response prediction. The effect of nonlinear boundaries of the structure is studied compared with the linear structure. It is found that stronger nonlinearities in the boundary make the system more stable. Analytical and simulation results show that nonlinear boundaries can significantly reduce the vibration and stress of flexible structures. It is important to point out that with the help of nonlinear boundaries, structural vibration and stress control can be achieved without altering the original structure.