The operation of a turbojet engine under windmilling conditions was analyzed and the relationship of engine speed and airflow has been derived. Replacing the efficiencies of compressor and turbine by a total pressure loss coefficient, the author was able to obtain the equations for matching the engine components and the universal curve of windmilling turbine characteristic based on theoretical analysis and experimental data. Based on the relations obtained, a simple method for calculating the windmilling characteristic of a turbojet engine was derived. In the absence of compressor and turbine characteristics under windmilling conditions, two approximate calculating methods were developed. The theoretical relations developed have been checked by experimental data. An example of the application of this approximate method is presented in the Appendix and the agreement between the prediction and experimental data is generally good.