Carbon fibres exhibit outstanding mechanical properties at low weight, which are maintained even at more than 2000 °C. They are the most favourable material for aironautic and space industrie. Thus the fibre properties and their structural development at high temperatures under load are of particular interest for technical applications. Within the frame of this work, two different testing devices are presented, which allow simultaneous loading in vacuum and in-situ structural investigation of carbon fibres by X-ray scattering. One of these devices was designed for testing of fibre-bundles in a laboratory X-ray device the other for testing of single fibres in a synchrotron radiation source. The development of the fibre structure is shown time resolved. The results obtained for structural and mechanical parameters allow to distinguish between different processes being responsible for stress graphitisation. A new model for restructurisation at temperatures below 1900 °C is proposed. Additionally, the thermal expansion of the carbon nanocrystallites within mesophase-pitch based fibres, studied in a synchrotron radiation source, is presented.