This paper presents a study of the effect of back pressure variation on mass flow rate and normal shock occurrence in a sonic nozzle. FLUENT software was used for numerical simulation and experimental measurements were carried out to verify numerical results nozzles of various designs (different divergence angles and throat diameters) were subjected to investigation. The simulation results indicate that both the divergence angle and the throat diameter markedly influence the performance of via the occurrence of normal shock in the divergent part of the nozzle. For the nozzle throat diameter range 0.558 mm to 3.175 mm and the back pressure ratio is 0.6. For this diameter range, the corresponding distance from throat at which a normal shock occurs is 1.3 to 1.7 throat diameters.