Alumina-bonded nano-polycrystalline diamond was synthesized by detonation sintering. The thermal stability, morphology and crystal structure are analyzed by thermogravimetric analyzer (TGA), scanning electron microscope (SEM) and transmission electron microscopes (TEM). Results show that the nano-polycrystalline diamond has good thermal stability in air with no obvious oxidation and good dispersity, the particle sizes of which are 0.5 to 0.7 μm. The nano-polycrystalline diamond is made up of large particles sized 100 to 200 nm, which are the aggregates formed by 2 to 10 nm alumina tightly bonded nano-diamond particles.