Granular diamond suitable for use as an abrasive grain for polishing is synthesized from gaseous phase by a process for the synthesis of granular diamond, comprising subjecting a mixed gas containing an organic compound and hydrogen to a treatment to form plasma and depositing diamond on substrate grains dispersed and fluidized in the resulting plasma space, characterized in that the substrate grains are dispersed and fluidized by the mixed gas fed at a flow rate of at least the terminal velocity of the grains and an AC or DC electric field is applied to a zone whose grain concentration in the space is in the range of 1 to 20% by volume.