The stability of masonry arches with an elastic jointing material is studied by considering the change in the balance of energy associated with possible modes of distortion in the arch configuration. The geometrical constraint on these modes of distortions are studied and the critical state of the arch is formulated and the critical load is analyzed. A simply supported masonry arch composed of three voussoirs is analyzed. Uniformly distributed load and the case where an additional point load is applied to the arch are considered. The implication of the instability to the arch failure is discussed.