The polymorphic forms of a new PDE-5 inhibitor DA-8159 were prepared and characterized by differential scanning calorimetry (DSC), powder X-ray diffractometry (PXRD) and thermogravimetric analysis (TG). Two crystal forms and one amorphous form of DA-8159 have been isolated by recrystallization and characterized by DSC, TG and PXRD. From the TG data it was confirmed that two crystal forms are neither solvates nor hydrates. The PXRD patterns of the two crystal forms were different. In the dissolution studies in simulated intestinal fluid at $37{pm}0.5^{circ}C$, the solubility decreased in the order of amorphous form > Form 1 > Form 2. After storage of 60 days, Form 1 was transformed to Form 2. Form 2 was not transformed. The amorphous form was transformed to Form 2 at 52% R.H. and 95% R.H., but it did not transform at 0% R.H.