Disclosed herein are a process for producing mixed crystals of disodium 5'-guanylate and disodium 5'-inosinate which comprises precipitating mixed crystals of disodium 5'-guanylate and disodium 5'-inosinate (I+G mixed crystals) by adding an aqueous mixed solution of disodium 5'-guanylate and disodium 5'-inosinate and a hydrophilic organic solvent at the same time into a crystallization vessel in such manner that the ratio of the hydrophilic organic solvent to the liquid phase in the crystallization vessel is maintained in a range of 30 to 70 vol %, as well as such process for producing I+G mixed crystals wherein said producing of I+G mixed crystals is carried out by seeding crystallization wherein crystals of 5'-IMP2Na or/and I+G mixed crystals are used as seed crystals. According to these production processes, the by-production of amorphous solids of 5'-GMP2Na which adversely affect separability of the crystals concerned, is inhibited, and therefore, such mixed crystals having a good separability without contamination of such amorphous solids, can be produced in a high productivity.