This paper describes influence of the molecular weight on thermally enhanced photoinduced reorientation of polymethacrylate films comprising 4-methoxyazo-benzene side groups, generated by an axis-selective Z-to-E photoisomerization using linearly polarized (LP) 633 nm He-Ne laser light and subsequent annealing. The polymers with number average molecular weight (M-n) between 4800-27000 were synthesized by the atom transfer radical polymerization (ATRP) method. Regardless of the molecular weight, high in-plane reorientation order parallel to polarization direction (E) of LP 633 nm light was accomplished when the exposed films were annealed in the smectic temperature range of the polymer films. However, degree of the thermal amplification of the photoinduced optical anisotropy depended on the molecular weight when the annealing temperature increased in the nematic temperature range.