The anhydrate and monohydrate of nitrofurantoin were characterized by X-ray diffraction analysis, infrared (IR) spectroscopy, thermal analysis and scanning electron microscopy. The X-ray diffraction patterns and the IR spectra of the anhydrate and monohydrate were significantly different, respectively. The differential thermal analysis (DTA) curve of the anhydrate showed endothermic and exothermic peaks at 272 and 276°C, respectively, with a loss of weight due to decomposition on the thermogravimetry curve. The DTA curve of the monohydrate showed an endothermic peak at 120-128°C with a loss of weight due to the dehydration of 1 mol/mol of crystal water, and endothermic and exothermic peaks at 273 and 276°C, respectively, accompanying decomposition. The initial dissolution rate of anhydrate and monohydrate in JP XI, 2nd fluid (pH 6.8) at 37°C were measured to estimate for solubility by using a rotating disk method. The solubilities of the anhydrate and monohydrate were estimated to be 47.8 and 27.4 mg/100 ml, respectively.