cBN-diamond composite sintered compacts (diamond content 15–70 wt %) were prepared by reaction sintering at 7–7.5 GPa and 1400–1700 °C for 10–30 min from the starting powder of the hBN-diamond system in the presence of 1 wt % NH4NO3 as a volatile catalyst. A fully dense sintered compact with 99% conversion from hBN to cBN was obtained at 7 GPa and 1700 °C after 30 min. An induced transformation from hBN to cBN seemed to occur on the surface of the added diamond seed crystals. Diamond seed crystals (about 30 wt %, grain size 0.2–1.5 μm) were found to be well-dispersed in the reaction-bonded cBN matrix. The Vickers microhardness of the sintered compact was 5100 kg mm−2. The contacts between diamond grains were observed in the sintered compacts containing diamond seed grains of more than 70 wt %. The toughness of the sintered compact tended to increase with decreasing diamond content and the grain size of seed crystals.