A study was conducted to develop and demonstrate a multi-objective design optimization method for rocket-based combined cycle (RBCC) engines. A method based on evolutionary algorithms (EAs) was developed and proved to solve the problem of discontinuous or concave Pareto fronts for mathematical programming techniques. The effects of the gas properties that depended on the primary rocket propellant combination and mixture ratio and on the ramjet equivalence ratio were considered for the investigations. It was assumed that the throat area A* of the primary rocket nozzle was choked and that a supersonic primary flow entrailed a subsonic secondary flow into a constant area ejector mixing duct. The conservation equations were also applied to the overall control volume consisting of the constant-area ejector mixing duct.