Cubic boron nitride (cBN) composites starting with cBN–Al mixtures were sintered on WC-16 wt% Co substrates under static high pressure of 5.0 GPa and at temperatures of 800–1400 °C for 30 min. Vickers hardness of the sintered samples increased with increasing cBN content, and the highest hardness of 32.7 GPa was achieved for the cBN–5 wt% Al specimens sintered at 1400 °C. The reactions between cBN and Al started to occur at about 900 °C, and the reaction products strongly depended on the Al content, sintering temperature, and Co diffusion from the substrates according to the x-ray diffraction (XRD) observations. The high pressure and high temperature in situ resistance measurement indicated that the reactions between cBN and Al could be completed in about 90 s when the temperature was higher than ∼1200 °C at high pressure. The cBN composite sintered at 1200 °C from a cBN–15 wt% Al mixture showed the best cutting performance.