A set of complete factorial design experiments on the vapour-phase synthesis of silicon nitride powders from SiCl₄ and NH₃ precursors has been carried out in a laboratory radiofrequency (rf) thermal plasma reactor. The system variables included plate power, flow rate of sheath gas and feed rates of reactants. Nitrogen content and yield of powder were selected as system responses. The factorial design experiments were followed by step-by-step design tests in order to find the optimum performance of the thermal plasma reactor. Synthesis conditions corresponding to the maximum of object function were established. Plate power and feed rate of SiCl₄ were found to be dominant system variables. The powder produced at the optimum conditions contained 38.2% nitrogen and 2.6% oxygen. The mean particle size of the mainly amorphous (<10%α-Si₃N₄) powder was 30nm. It consisted of spherical, uniform, loosely agglomerated particles.