The Electron Cyclotron (EC) Heating system for the EU-DEMO tokamak, designed within the EUROfusion consortium activities as a device to demonstrate the feasibility of a Fusion Reactor, will inject power in different plasma locations to provide a series of tasks, including plasma current ramp-up and ramp-down, central (Bulk) Heating (BH), Neoclassical Tearing Modes (NTM) stabilization, Radiative Instability (RI) control. For fulfilling the tasks, a large amount of mm-wave power at different frequencies has to be generated, transmitted for more than hundred meters, and finally injected into the torus. The transmission line concept being developed is capable to carry multiple frequencies at the same time and can be adapted to final changes of the physics baseline made for optimizing the DEMO performances. The transmission line is arranged in a modular way, in order to be tailored to the final requests of power and reliability, determining the number of gyrotron sources and used ports. The basis of the organization of the system is the “cluster” of sources that share a single multi-beam (MB) transmission line, with one or two clusters sharing the same EC launcher in the equatorial port of DEMO. In this work the basic design and the layout of the transmission line is described, as well as the options for the routing, the MB mirror layout and the grouping in clusters based on reliability calculations. It also describes the work on the broadband polarizers with minimal losses, the coupling of the TEM00 beams to the waveguides, the MB mirror cooling design and the electromagnetic evaluations of transmitted beams to evaluate the coupling losses due to mirror thermal and gravity deformations.