We demonstrated advanced techniques for control of gas flow in a reactor in order to achieve good etch depth uniformity for large area GaAs etching. It was found that a finite difference numerical method was quite useful for simulation of gas flow distribution in the reactor for dry etching of GaAs. The experimental results in BCl/N/SF/He ICP plasmas confirmed that the simulated data fitted very well with real data. It is noted that a focus ring could help improve both gas flow and etch uniformity for large area GaAs plasma etch processing. The simulation results showed that optimization of clamp configuration could decrease gas flow uniformity as low as ±1.5% on a 100 mm (4 in.) GaAs wafer and ±3% for a 150 mm (6 in.) wafer with the fixed reactor and electrode, respectively. Comparison between simulated gas flow uniformity and real etch depth distribution data confirmed that control of gas flow distribution in the chamber would be significantly important in order to achieve excellent dry etch uniformity of large area GaAs wafers.