The chaotic oscillation of a buckled beam under sinusoidally varying and static constant transverse external forces is investigated. A harmonic balance method and a direct numerical integration are applied to a Duffings equation model of the buckled beam. For a small transverse constant force, there exists three static equilibrium points, and the near contact between the two orbits of a stable and an unstable limit cycle in the phase plane can predict the onset of chaos. For a large transverse constant force, there exists only one static equilibrium point, but there may exist three different dynamic response amplitudes due to non-linear resonance phenomena. The near contact between two vibration regions of a stable and an unstable limit cycle can predict the onset of chaos. © 1991.