GMP (Guanosine-5'-monophosphate) is valuable flavoring agents for foods, beverages and seasonings. GMP is produced through hydrolysis of nucleic acid by means of enzyme. GMP is then separated by conventional methods combined with ion exchanger, adsorption, chromatography, etc. In present commercial process, above complicated processes for producing the crystalline form except in the amorphous form requires the large energy consumption and huge amount solvent and thus high operating cost. Most works on GMP reported fermentation and purification of GMP, but a few works investigated methods to upgrade the GMP particle. In addition, the mechanism of formation of GMP particle has never been explored. In this paper, we studied on the preparation of crystalline GMP particle using crystallization process. This research investigated the effect of the experimental variables on the characteristics of crystalline particle, such as purity, morphology and crystal size. Nucleation and growth of GMP was monitored by on-line measurement during crystallization.The time to be consumed to convert amorphous into crystalline particles depended on seed amount, PH, feed rate of methanol, methanol/solution rate, temperature, etc. NaCl as an additive activated formation of the crystalline particle.