Sterilizing or disinfecting narrow-lumen instruments and endoscopes is a highly difficult process. Although plasma sterilization is promising and effective against a broad spectrum of microorganisms, its effectiveness is dependent on the density of the excited particles generated by plasma that attack the microorganisms through erosion. A numerical simulation method is used to study the transport and density of the excited nitrogen molecules in a narrow lumen. The gas properties are calculated using the gas kinetic theory, and different lifetimes are assumed for excited molecules. The simulation results show that the survival of excited particles in a lumen increases more than 10 times when the pressure is reduced by a factor of 10. It is highly recommended that the plasma be operated at lower pressure to increase the sterilization efficiency of narrow lumen devices.