The ITER Electron Cyclotron Heating (ECH) system Transmission Lines (TL) require highly polished copper mirrors on miter bends (both 90° and 140°) to direct microwaves from their origin to the tokamak. This will result in substantial heat dissipation on the miter bends and mirrors and will require water cooling in order to achieve long pulse operation. Analysis and optimization of the cooling design for the 140° miter bend assembly used ANSYS® Multiphysics™ software to develop and verify the fluid, thermal, and structural behavior of the mirror and miter bend assembly. Simulation model choices included a thermo-mechanical model of the mirror-only, a thermo-mechanical model of the miter bend assembly, and a thermo-mechanical model of the mirror with coolant. These analyses revealed an optimal solution that uses a major-axis cooling channel configuration for the 140° miter bend to meet the design criteria (e.g. structural stresses, mirror deflection, vacuum seal, coolant temperatures and pressures).