The effect of high molecular weight additives (PVA and gelatin) on barium sulfate precipitation in an MSMPR reactor is investigated. The impeller speed is varied from 0 to 1200 rpm and the additive concentration in the bulk solution is increased up to 5.0 g/1. As the additive concentration is increased, the particle growth rate decreases and the nucleation rate increases. However, the particle morphology is not changed by the additives. The experimental results are explained qualitatively by supposing that the additive is adsorbed on the particle surface and inhibits the mass transfer to the surface.