Dilute GaNxAs1-x thin films with N concentration from 0.0% to 3% have been grown by molecular beam epitaxy. Piezomodulated reflectance (PzR) spectra of these thin films have been measured at room temperature, and optical transitions in PzR spectra have been well resolved. The N-related transition of E1+Δ1+ΔN had been observed in PzR spectrum of GaN0.005As0.995 and GaN0.01As0.99 films. The well resolved split between heavy hole and light hole of Γ valence band is shown in PzR when the N doping level is up to 1%. The N concentration dependence of transition energies supports the model that both E+ and E* originate from the L conductive band at room temperature.