The hypothetical nanometer sized charged diamond clusters, which were predicted in the charged cluster model [Journal of Crystal Growth 62 (1996) 55], were experimentally confirmed in the hot filament diamond chemical vapor deposition (CVD) process by the combination of a Wien filter and an ion mobility analyzer. Under typical processing conditions for CVD diamond, negatively charged clusters composed of hundreds of atoms were measured. The mass distribution showed a peak at similar to 3000 atomic mass unit with a gas mixture of 1.5% CH4-99% H-2. The number density of the negatively charged clusters was estimated to be similar to 10(6) mm(-3).