2,2':6',2 ''-Terpyridine (terpy) is interesting photophysical molecule which exhibits long-lived phosphorescence. And more long-lived phosphorescence is expected in a solid state. Until now, two crystal structures were reported for terpy. However, details of photophysical properties of the crystals were seldom discussed. We show new method for controlling the two polymorph of terpy using magnetic-field and possibility for the development of magnetic-tunable organic phosphorescent materials is discussed. Under normal conditions, the orthorhombic crystals were obtained. Under magnetic field above 5 T, however, the monoclinic crystals were precipitated. The monoclinic crystals exhibited room temperature phosphorescence. The lifetime of phosphorescence was increased with increasing magnet intensity.