Sintering behaviors of fine-grained cBN–10wt.% Al 3.21 Si 0.47 system under high pressure of 5.0GPa and temperatures up to 1600°C were investigated. By analyzing the phase components of sintered samples through x-ray diffraction (XRD), and detecting the in-situ electrical resistance signal during high pressure and high temperature (HPHT) sintering, we suggested the reactions between cBN and Al 3.21 Si 0.47 happened at about 600°C. Between 600°C and 900°C, AlN, AlB 2 and Si formed, while above 1000°C, the content of AlN, AlB 12 and Si phases increased with AlB 2 disappearing. Scanning electron microscope (SEM), Vickers hardness tests as well as cutting performance tests showed that the well-sintered samples in cBN–10wt.% Al 3.21 Si 0.47 with homogeneous microstructure and best mechanical performance could be obtained at the sintering P–T condition of 5.0GPa, 1400°C for 10–15min.