Thick free-standing boron-doped diamonds were prepared by microwave plasma assisted chemical vapour deposition. Samples with a final thickness close to 5 mm and with lateral dimensions 25 × 25 mm were produced. The thermal shock resistance of the material was tested by exposure in the JUDITH electron gun to reproduce the conditions expected in a fusion reactor during plasma instabilities. Boron-doped diamond is shown to exhibit thermal shock resistance far better than current material of choice for a fusion reactor like tungsten, with no surface damage after exposure to 100 cycles at 2.5 GW m−2 for 5 ms.