In order to study the buckled states of an elastic ring under uniform
pressure, Tadjbakhsh and Odeh [14] introduced an energy
functional which is a linear combination of the total squared curvature
(elastic energy) and the area enclosed by the ring. We prove that the
minimizer of the functional is not a disk when the pressure is large,
and its curvature can be expressed by Jacobian elliptic
$\cn({}\cdot{})$ function. Moreover, the uniqueness of the minimizer
is proven for certain range of the pressure.