A new linear instability mechanism of curvature origin is established for a vortex ring. The curvature effect reduces O(2) × SO(2) symmetry of a circularcylindrical tube to O(2), and fuels a pair of Kelvin waves whose azimuthal wavenumbers on the core are separated by one. For Kelvin's vortex ring, the growth rate and eigenfunctions are written out in closed form. In the inviscid case, the curvature effect dominates over the elliptically straining effect, but the former suffers from enhanced viscous damping. There are numerous excitable modes. As a first step toward an understanding of the route to a matured stage, we derive equations for weakly nonlinear evolution of amplitudes of the curvature instability. Our direct numerical simulation successfully captures the elliptical instability.