The Polymorphism of (BEDT-TTF)(2)I-3 was studied with X-ray diffractmetry. The observation of solution-mediated transformations clarified that the beta -form is thermodynamically more stable than the alpha -form at least in the temperature range between 0 and 50 degreesC. The metastable oc-form preferentially crystallized with increasing oxidation current or lowering the growth temperature. This polymorphic behavior was discussed in term of supersaturation using the classical equation of the nucleation. Using the information of the polymorphism, large single crystals of the alpha- and beta -forms were selectively grown under the designed conditions.