Diamond-SiC composites were produced from diamond and Si powders by reaction synthesis using a hot isostatic pressing (HIP) technique. At an HIP condition of 1450°C and 100MPa, which pressure is much lower than that of the diamond stability field, diamond powders react with molten Si to form well-sintered diamond-SiC composites. Using the cubes of the composites thereby fabricated, an application to the second stage anvils in the Kawai-type multi-anvil high-pressure apparatus was attempted. Because the diamond-SiC composites are transparent to X rays, the present anvils can be employed not only for conventional energy dispersive X-ray diffraction studies but also for angle dispersive diffraction and radiographic studies that need a larger window for X-ray images. In this paper, our recent advances in the HIP production of diamond-SiC composites and their applications to high-pressure anvils for the Kawai-type multi-anvil high-pressure apparatus were reported.