In the fabrication of nitride-based laser, the cavity plays a vital role in order to enhance the fundamental modes while suppressing the higher order modes. The critical problem faced between the designer and the grower is the idealistic approach in the design laboratory, which more often cannot be met in the fabrication lines. This is because the design does not take into account the limitation in the growth chamber in achieving the targeted mirror thickness and material composition. This paper discusses methods of designing DBR mirrors for vertical cavity surface emitting GaN based lasers. Our design considers variance in the thickness and material composition of the mirrors. The simulation results are given for the investigation of several properties of DBR mirror, as well as a comparison of same types of symmetric DBR mirror.