Crystals of (2R,4R)-monatin potassium salt which exhibit an X-ray powder diffraction pattern having characteristic peaks at diffraction angles 2.theta. of 5.5.degree., 7.2.degree., 8.1.degree., 8.9.degree., and 16.3.degree., by powder X-ray diffraction (Cu--K.alpha. radiation), are superior in sweetness, preservation stability, solubility, and color stability.