<p>  Single-crystal AlN/diamond heterojunction with high-density interface hole channel was successfully obtained by metal-organic vapor phase epitaxy. AlN layer was epitaxially grown on hydrogen-terminated (H-)diamond(111) substrate. Thermal treatment of diamond substrate was carried out under hydrogen and ammonia mixture environment at 1250 ℃ just before AlN growth. This thermal treatment led to surface sheet hole density as high as ~1.0 × 10<sup>14</sup> cm<sup>-2</sup> without structural reconstruction of diamond surface. In addition, the use of smaller off-cut angle (0.20 ± 0.25°) H-diamond(111) substrate combined with this treatment enabled to obtain single-crystal epitaxial AlN layer, which simultaneously acted as passivation of the surface hole channel with such a high density. The AlN/H-diamond(111) hetero-junction revealed type-II staggered energy band configuration with valence band offset of ~2.0 eV, which is suitable for the fabrication of p-channel field-effect transistor using AlN-gate-insulator/ diamond heterojunction. These results are promising for the development of AlN/diamond hybrid power electronic devices.</p>