The effect of microwave radiation on anti-solvent crystallization was investigated by in situ observation of nanoparticle size in a reactor. The precipitation of sodium chloride by the addition of ethanol was accelerated by the increase in molecular diffusion caused by microwave radiation. The microwave power controlled the particle size because increasing the power increases the temperature, which leads to the dissolution of the precipitate. The salt concentration in the solution, and the anti-solvent concentration and volume were important factors in anti-solvent crystallization under microwave radiation. (C) 2014 Elsevier B.V. All rights reserved.