A numericalmethod in the time domain is introduced to assess the stability with respect to the nozzle’s geometry in liquid rocket engines. A boundary condition for the injector plane in a combustion chamber is presented, where the mass flow fluctuations are set to zero. Thus the acoustic flux over the injector plane is zero even when there is mean flow normal to the boundary. Two ways of assessing the nozzles stability are shown. The first is to characterize the decaying of pressure oscillations means by the decay coefficient and the second is to determine the nozzle admittance. The procedures for obtaining both are described and the results are compared with the available experimental and analytical data.