The M1 mirrors are part of the in -vessel quasi -optical Electron Cyclotron beam propagation system of the DEMO nuclear fusion reactor. These four mirrors are all non -movable and with a plane reflecting surface. Based on their functionality they are divided into two groups, the Mid -Steering Antenna (MSA) and the Multi -Beam Mirror (MBM) antenna. The MSA is responsible for controlling the neoclassical tearing modes, while the MBM is available for plasma bulk heating, current drive and thermal instability control. These mirrors are designed to withstand the thermal loads generated during operation: neutron heating, ohmic losses and stray radiation caused by the mm -wave beam reflection. To fulfil the design requirements, efficient heat removal strategies based on water cooling principles minimizing pressure drop are implemented. This paper presents the pre -conceptual design of the MBM and MSA M1 mirrors, validating the structural and geometric boundary conditions, as well as the appropriate selection of materials. Using FEA tools, thermalhydraulic and thermo-mechanical simulations were performed with the available thermal loads to optimize the cooling parameters and perform a first stress assessment on the two M1 mirrors.