The optical properties and fabrication of ultrathin GaN-based microcavities grown on silicon substrates are described. The epitaxial part of the optical cavities, consisting of lambda/2 GaN layer above a 3-period epitaxial Bragg mirror, is sandwiched between two silica/zirconia mirrors. At a suitable point in the fabrication process the silicon substrate was selectively removed using via holes. The cavity mode and excitonic resonance are observed by reflectivity at low and room temperature, demonstrating a quality factor of similar to 125. The dispersion of the modes and their linewidth is measured using angle-resolved reflectivity and successfully modelled using transfer matrix simulations.