A scattered radar signal by a jet engine inlet involves engine information of the target. The jet engine RCS due to the engine inlet is therefore significant in radar target recognition. Accordingly, several accurate analyses of the jet engine RCS have been reported. Reduction of the jet engine RCS is also required for stealth technology. We suggest a new approach for jet engine RCS reduction that involves the insertion of many small corrugations into the jet engine inlet. The proposed jet engine structure is assumed to be a circular waveguide cavity with many corrugations. The structure is analyzed by combining a mode matching technique with a scattering matrix analysis. The proposed closed-form RCS solutions of the corrugated engine structure are validated with a MoM simulation using FEKO. The proposed closed-form solution has numerical efficiency and rapid convergence. Using the characteristics of the solution, we simply apply a genetic algorithm (GA) to optimize the structure of the corrugations in terms of minimizing the RCS.