The mesomorphic phase of poly(butylene-2,6-naphthalate) (PBN) has been studied using wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). It was found that PBN showed a mesomorphic phase when it was rapidly quenched from the molten state to 0 degrees C. The mesomorphic PBN was nearly transparent solid in appearance and its structure in WAXD scale was an intermediate one between amorphous and crystal having smectic periodicity. With increasing the temperature, a portion of amorphous remaining in the quenched PBN transformed into the mesomorphic phase above 48 degrees C, and then the mesomorphic phase began to transform into the alpha-crystal above 71 degrees C. We propose that the mesomorphic phase of PBN has a glass transition temperature T-g(meso) at 65 degrees C besides the normal glass transition temperature T-g for the isotropic glass at 41 degrees C.