De Laval nozzle is a converging-diverging nozzle which can convert the chemical energy with high pressure into kinetic energy with high velocity and low pressure. The scheme of change of speed and pressure within the nozzle depends on the change of the cross-section area of the nozzle. The nozzle is used in jet engines. This study is concerned with the study of the performance of the work of de Laval nozzle, two models were designed; the first model: when the total pressure (gauge) of air at the entrance is equal to 9 bar and the second model: at the total pressure (gauge) of the entrance is equal to 11 bar. The total pressure in the two models was decreased, and the change in the Mach number was observed at the outlet of the De Laval nozzle. The models were designed and analyzed in the ANSYS Fluent program. The total air pressure changes at the nozzle entrance and their effect on changing the Mach number at the exit were studied. The results obtained from the ANSYS. Fluent program was compared with the theories of gas dynamics.