  An integrable spin-ladder model with nearest-neighbor exchanges and
biquadratic interactions is proposed. With the Bethe ansatz solutions of the
model hamiltonian, it is found that there are three possible phases in the
ground state, i.e., a rung-dimerized phase with a spin gap, and two massless
phases. The possible fixed points of the system and the quantum critical
behavior at the critical point $J=J_+^c$ are discussed.
