We report here on the fabrication of a guided-mode resonant III-nitride grating reflector on a silicon substrate. In addition to compensating the residual stress of III-nitride layers, hafnium oxide (HfO2) film also serves as a hard mask during inductively coupled plasma reactive ion etching of III-nitride layers. The silicon substrate underneath the III-nitride gratings is etched and thus the III-nitride gratings are released and freely suspended with air as low refractive index materials on the top and bottom. The guided-mode resonances that are affected by the grating period, duty ratio, polarization and effective refractive index are experimentally characterized in the reflectance measurements. These works open the possibility of fabricating resonant III-nitride structures on the silicon substrate for further tunable III-nitride optical devices and integrated optics.