A joining of Be to ferritic-martensitic steels (FMS) is an essential process in the fabrication of ITER test blanket module (TBM). The diffusion barrier layers together with the coated interlayer were applied to the HIP joining of Be and FMS in order to develop the interlayer technology for the fabrication of ITER TBM. Multiple layers formed due to an excessive diffusion of elements in the interface region in the absence of a diffusion barrier layer. Such a complicated interface structure consisting of brittle phases in part would be very prone to fracture even at low stress levels. A Cu foil or a HIPed CuCrZr layer applied as a diffusion barrier was effective to retard the diffusion between Be and FMS. It was revealed that the diffusion barrier layers helped to improve the joining properties by reducing the possibility to form diffusion layers in the interface, which made the Be/FMS joint have an appreciable joining strength.