Films of AlGaN and GaN are used as a Distributed Bragg Reflector (DBR) mirror for light emitting diode (LED) and vertical-cavity surface-emitting laser (VCSEL) type of laser. In this paper, we report the influence of different substrates on the reflectivity of DBR structure by using three different substrates, sapphire, silicon carbide and silicon. The DBR structure and optical properties of the films have been studied using the transfer matrix method (TMM). Better characteristics are obtained when Si substrates are used as compared to conventional Al2O3 substrates. This suggests that Si is a very promising substrate for GaN-based DBR mirror for blue laser diodes...