The novel feature of the co-turbojet engine is the rotation of tne casing of the. high pressure compressor. This casing co-rotates, at lower speed, with the high pressure rotor of the turbojet engine, hence the name co-turbojet. The compressor casing"derives power from the high pressure air stream passing through the compressor and this power is used to drive tine low pressure compressor, which is mechanically coupled to it. This paper studies the consequencies of such basic modification in respect to both engine design and performance. The paper presents what is thought to be a suitable meChanical arrangement of this engine. With this arrangement, it is possible to dispense with the low nressure turbine and its rotor by adding one stage to the nigh pressure turbine. Consequently a great saving in engine length and weight can be achieved. Avoiding problems and complexity, resulting from having two shafts rotating one inside the other at different varying speeds, are some of the attractive features of the coturbojet engine. Basic thermodynamic analysis of the engine are carried out to determine the critical parameters affecting its performance. The paper formulates torque and power constraints of the engine as well as components speed relationship. Performance of engine components in view of the likely different loading is disscussed. Among the attractive characteristcs of the engine is the control of thrust by means of speed control of the rotating compressor casing. The increased flexibility of engine control adds to its merits.