The equilibrium state and crystal growth of (BEDT-TTF)2I3 were investigated. Measurements of the temperature dependence of solubility and analyses of the components in the solution were carried out. The analysis of the solution showed that the crystals decompose into neutral BEDT-TTF and I2 molecules. As to the polymorphism, it was clarified that the β-form is more stable than the α-form in the temperature region between 0 and 50°C. Using these information, we succeed in growing single crystals form BEDT-TTF and I2 and in control of the crystallizing polymorph. The polymorphic behavior was explained in terms of supersaturation as a driving force of crystal growth.