Standard variable angle spectroscopic ellipsometry (VASE) has been employed to study the ordinary optical dielectric response of hexagonal gallium nitride (GaN) thin films—an important material for blue and ultraviolet light emitting device applications. The GaN films were grown by molecular beam epitaxy on c-plane sapphire substrates (α-Al2O3). Room temperature isotropic and anisotropic mode VASE measurements were made at angles of incidence between of 20° and 80°. Evidence of anisotropy was observed from the anisotropic mode measurements, reflecting the nature of wurtzite crystal structure of GaN. The sizable off-diagonal elements (Aps and Asp) of the Jones matrix indicate that the optical axis of the c-plane sample are slightly off from the surface normal due to a small miscut of substrates. VASE data simulations by isotropic and anisotropic models indicate that the anisotropic effect on both diagonal and off-diagonal elements of the Jones matrix can be minimized to a negligible level at small angle of incidence. Thus the ordinary optical dielectric functions (E⊥) are precisely determined by the isotropic mode VASE measurements at angles of incidence between 20° and 40° in the range of 0.75–6.5 eV. The VASE data were analyzed by a model dielectric function based on the GaN critical point structure, which allows for a nonzero extinction coefficient k below the band gap. The thicknesses of these GaN films are accurately determined via the analysis as well. © 2000 American Institute of Physics.