The goal of this work is to study the sublimation growth of GaN and the evaporation behaviour of GaN powders. The reactor design and growth parameters, i.e. gas pressure, gas flow, growth temperature and hot zone design together with the RF heating system, were optimized by using numerical simulation and thermogravimetric (TG) studies. Numerical modelling using the software packages CrysVUn and Crys-MAS was used to adjust the flow pattern with respect to a maximum species transport towards the growth interface and to find an appropriate pressure range.