Abstract This study investigates the optical anisotropy spectrum in the R-plane (i.e., the [ 10 1 ¯ 2 ] -oriented layer plane) of GaN/Al0.2Ga0.8N quantum wells of different widths. The optical matrix elements in the wurtzite quantum wells are calculated using the k ⋅ p finite difference scheme. The calculations show that the valence band mixing effect produces giant in-plane optical anisotropy in [ 10 1 ¯ 2 ] -oriented GaN/Al0.2Ga0.8N quantum wells with a narrow width. The nature of the in-plane optical anisotropy is found to be dependent on the well width. Specifically, it is found that the anisotropy changes from x ′ -polarization to y ′ -polarization as the well width increases.