Diamond (carbon) and silicon powders were mixed and HIPed under temperatures of 1300degrees-1500degreesC and pressure at 50 MPa for 30 min., When heated at >1300degreesC, the products were > 90% sintered compacts. Density and bending strength were measured. The highest values of 3.3 g/cm(3) and 750 MPa were obtained when the starting material was a mixture of fine and coarse-grained diamond and silicon powder. The photomicrograph of polished surface of the product revealed that it consisted primarily of two types of substances with few pores. XRD showed the coexistence of diamond and SiC. No trace of conversion reaction from diamond to graphite was seen, although the sample was treated under conditions in which diamond was thermodynamically metastable. The summarized results suggest that the HIP process can be a useful way to synthesize diamond/SiC composites.