Abstract: This paper presents some results of optimization of operating modestransfer in a Supercharged Ejector Ramjet (SERJ) specified for first stage of a two-stage-to-orbit space vehicle. The SERJ consists of four subsystem:Fan, Ejector, Ramjet and Air liquefaction subsystem used as a heat exchanger. Three factors are considered for estimating propulsion system installed thrust; Internal and external performances of the inlet and the nozzle; power and air extractions from the engine. An objective of minimization is the minimum of the fuel consumption, which is attained by varying the transition conditions of different engine operation modes. Then the optimum transition corditions can be obtained between SERJ and fan ramjet modes, fan ramjet and ramjet modes, ramjet and ejctor ramjet modes, ejector ramjet and rocket modes.