Abstract: The characteristics of the steady(surge-free)operating margin of a dual-rotor compressor in a turbojet engine are discussed, and the practical technical requirements are given for engineering evaluation of the steady operating margin. For the dual-rotor compressor, there is a great influence of the ratio n2/n1(H. P. rotor speed to L.P.rotor speed)on the steady operating margin. In order to guarantee the steady operation of the compressor in the whole flight envelope, it is necessary to check the surge-free operating margin not only in a specified range of the L.P. rotor speed but also in a specified range of n2/n1. A numerical example of a turbojet engine is given to illustrate the above requirements.