A hybrid rocket motor is an intermediate choice between solid and liquid propulsion systems. The use of paraffin as fuel increases the regression rate because of the formation of droplets originated from a process of hydrodynamic instability on the oxidant-fuel interface. Chamber pressure is measured in a laboratory test with a hybrid configuration that uses paraffin and gaseous oxygen. In addition, a numerical solution of the continuity equation is applied to the combustion chamber, using an algorithm that takes into account physical and thermo chemical properties of the propellants as well as the internal geometry of the motor. Finally, numerical and experimental results are compared.