On the basis of Rigorous Coupled Wave Analysis( RCWA), an array of freestanding non-periodic GaN gratings were proposed to control the phase of an incident beam in the visible wavelength region. Firstly, the Finite Difference Time Domain(FDTD) method was used to calculate the optical responses of non-periodic GaN gratings based on the parameters such as period and duty cycle. Then, a double-sided process and nitride back thinning technology were developed to fabricate freestanding non-periodic GaN gratings on a GaN-on-silicon platformand to control the different phase shifts of incident light. Finally, the optical performance of non-periodic GaN gratings was experimentally demonstrated by angular resolved micro-reflectance spectra and photoluminescence spectra. The angular resolved micro-reflectance spectra show that the optical performance of the freestanding non-periodic GaN gratings is in good agreement with that of the theoretical simulations by the FDTD method. The photoluminescence spectra indicate that photoluminescence (PL) intensities of the GaN gratings are greatly improved as compared to that of the GaN-on-silicon, and their emission peaks are from 364.3nm to 378.7nm. Moreover, experimental results give that the incident angular tolerance of the GaN gratings is -25°-25° in the visible wavelength region. In conclusion, the gratings are helpful for the improvement of the light extraction efficiency.