4-5　nm　nanodiamonds　tend　to　self-assemble　into　100-200　nm　nanodiamond　aggregations　and　furthermore　nanodiamonds　in　the　early　stages　of　the　detonation　process　present　fantastic　twinned　morphologies,　indicating　that　there　are　strong　interactions　among　these　nanodiamonds.　Herein,　electrostatic　interactions　and　van　der　Waals　interactions　between　two　nanodiamonds　are　explored　using　DFT　computations　in　conjunction　with　Monte　Carlo　molecular　simulations.　It　is　indicated　that　the　van　der　Waals　forces　are　much　stronger　than　the　electrostatic　forces　for　the　unsaturated　nanodiamonds.　More　importantly,　two　assembly　features　are　exposed　as　follows:　assembly　has　a　preferential　face-to-face　orientation;　assembly　has　a　strong　binding　energy　comparable　to　the　dissociation　energy　of　C-C　bonding,　which　is　116.1　kcal　mol(-1)　for　a　2.48　nm　truncated　octahedral　nanodiamond.　The　results　suggest　that　the　strong　forces　holding　the　nanodiamond　aggregation　almost　certainly　attribute　to　the　proposed　strong　van　der　Waals　forces,　which　is　of　great　importance　in　understanding　the　aggregative　properties　of　nanodiamond.