  The entanglement perturbation theory is developed to calculate the excitation
spectrum in one dimension. Applied to the spin-$\frac{1}{2}$ antiferromagnetic
Heisenberg model, it reproduces the des Cloiseaux-Pearson Bethe ansatz result.
As for spin-1, the spin-triplet magnon spectrum has been determined for the
first time for the entire Brillouin zone, including the Haldane gap at $k=\pi$.
