Ti₃SiC₂ and Ti₃AlC₂ are two promising binders for the ultrahard composite polycrystalline cubic boron nitride (PcBN). In this study, the cBN composites with Ti₃SiC₂ and Ti₃AlC₂ binders with different binder contents were synthesized under the same high pressure (~5.5 GPa) and high temperature (1350 °C) conditions. Their mechanical properties were measured separately, including Vickers hardness, bending hardening, and compression hardening. Together with XRD results, the phase compounds were also investigated. The decomposition and reaction processes were affected by binder content. Some new compounds formed during sintering, these compounds were TiC, TiSi_2, SiC, TiB₂, SiB₄, TiB₂, and TiC_0.7N_0.3 in Ti₃SiC₂-cBN composites, as well as TiC_0.7N_0.3, TiB₂, and AlN in Ti₃AlC₂-cBN composites. The microstructure of the cracks surface was obtained after the bending tests, and was used to further investigate and compare their crack mode by SEM. The crack surface profile and elementary analysis on the oxidative surface were also discussed.