The gases behave differently than the substances in solid and liquid state, they do not have a steady volume and are completely formed by the tank in which they are enclosed. Gas state can be fully determined with three parameters: pressure, temperature and volume. During the motion of the gas (flow) another parameter appears, that is a velocity . When flowing through the nozzle, the gas speed increases due to the drop in pressure and temperature, i.e. conversion of pressure and heat energy through enthalpy drop into kinetic energy. The nozzles also have the opposite effect of converting the kinetic energy into pressure, which depends on their shape and the gas input state. The shape of the nozzle is characterized by a cross-sectional change along the entire length, accordingly divided into three basic groups: convergent, divergent and convergent-divergent. Convergent nozzles accelerate subsonic and decelerate supersonic flow of gas. The cross-sectional area is continuously decreasing towards the exit. The best example is a reaction turbine. The cross-sectional area of a divergent nozzle continuously increases from the inlet to the outlet. These nozzles decelerate the subsonic and accelerate the sonic and supersonic flow of gas. They are the key element of all the dynamic compressors. The nozzle that is again expaneded after its narrow cross section is called the de Laval nozzle, and its function is to achieve supersonic speeds with higher thermal drops. It is applied to most impulse turbines and rocket jet engines as a drive nozzle in order for them to achieve the maximum speeds. The paper presents the applications of all three frorms of nozzles in power plants as well as the phenomenoms that occur during the flow.