To study the feasibility of producing nanoparticles of organic pharmaceuticals using a novel high-gravity reactive precipitation (HGRP) technique, reactive precipitation of benzoic acid as a model compound was carried out in a rotating packed bed under high gravity. The main factors such as the rotating bed speed, concentration and volume flow rate of the reactants (sodium benzoate and HCl) affecting the particle size of the precipitate were studied. Particle size was measured by transmission electron microscopy. Benzoic acid was precipitated as nanoparticles as fine as 10nm. The particle size was decreased with increasing rotating bed speed, concentration and volume flow rate of the reactants. The formation of ultrafine particles was due to intensified micro-mixing of reactants in the rotating bed to enhance nucleation while suppressing crystal growth. The results have demonstrated the feasibility to produce nanodrugs by the principle of acid-base precipitating reaction using HGRP.