Very fine diamond was synthesized under hydrothermal conditions, with new bond formation among diamond particles, in 10M-NaOH solution at 573K, 1GPa, for 24 hours. This new bonding material is formed by carbonization, from chlorinated hydrocarbon such as dichloromethane and 1,1,1-trichloroethane, under alkaline hydrothermal conditions. The carbonized material forms a new bond between diamond particles by the release of hydrogen-chloride using a hydrated diamond powder particle as the starting material. With pellet of compacted diamond, the platy materials were produced. These materials were diamond by micro-XRD analysis. New growth materials between compacted diamonds, and each diamond particles were bonded. Therefore, it is possible that compacted diamonds is sintered by carbon-bonds under hydrothermal synthesis. Hydrothermal hot-pressing is technic that materials as glass, calcium silicate and calcium carbonate is solidified. Compacted materials is deformed under hydrothermal conditions, and materials is solidified strongly by alterative materials. This technic can be applied to diamond sintering.