The optical polarization of Si-doped AlxGa1-xN epitaxial layers (x=0.37–0.95) has been studied by means of photoluminescence (PL) spectroscopy. The predominant polarization component of the band-edge PL switched from E⊥c to E||c at an Al composition between 0.68 and 0.81. This critical Al composition was much higher than in previous reports for AlGaN epitaxial layers. In addition, the predominant polarization in Al0.55Ga0.45N epitaxial layers switched from E⊥c to E||c with increasing Si concentration. Therefore, the topmost valence band changed from the heavy-hole band to the crystal-field split-off-hole band with decreasing in-plane compressive strain induced by Si doping.