The stellarator W7-X which is currently under construction in Greifswald, Germany, will be equipped with a powerful ECRH system, working at 140 GHz. The ECRH system is designed to operate in the continuous wave (CW) regime. The millimetre (mm)-wave power will be generated by 10 gyrotrons delivering 1 MW each and will be transmitted from the gyrotron hall to the machine via a fully optical system. The mirrors (more than 160) are water cooled and can be adjusted by step motors remotely. The transmission line consists of a single-beam waveguide part (SBWG) and a multi-beam waveguide part (MBWG). In the SBWG part (length appr. 10-15 m), close to the gyrotrons, the mirrors are related to the individual tubes. Two confocal MBWG lines (length appr. 40 m) support 5 beams simultaneously each. At the stellarator the beams are separated again and launched by individual antennas to the plasma. To reach high transmission efficiency, the design of the mirrors must guarantee a mechanically stable surface under the heat load imposed by the ohmic loss of the mm waves. The design consists of a 60 - 70 mm thick honeycomb structure from stainless steel ... mehr and a thin (2 mm) sheath of electro-formed copper on the mirror surface. Optimized cooling channels are milled directly below the copper in the stainless steel structure and form one or several spirals going from the centre to the edge of the mirror. According to thermo-mechanical calculations, a very low thermal deformation is obtained with this design, even during the transient phase in the first minute after power switch-on. This behaviour has been confirmed experimentally in a test set-up with a MBWG type mirror. It turned out, that deformation under application of a heat load equivalent to the absorbed power from a 1 MW mm-wave beam is in the tolerable range. Water cooled mirrors of the SBWG type have been used successfully during conditioning of a prototype gyrotron with a mm-wave power of 740 kW and a pulse length of up to 100s. To test and optimise in-situ beam and power measurements of the CW ECRH system on W7-X these mirrors are equipped with different diagnostics tools. We use a grating coupler, a waveguide coupler integrated into the surface of a mirror and sensors for calorimetric measurement of mm-wave absorption. Measurements and results on the deformation of mirrors for CW high power application in the mm-wave range will be presented and problems of the actual design will be discussed. This work has been performed in the frame of the project PMW, ECRH for W7-X hosted at FZK Karlsruhe (collaboration between FZK Karlsruhe, IPP Garching and Greifswald, and IPF Stuttgart).