It has been known that twin symmetry occurs in small metal crystals and it could lead to ultrahigh mechanical properties. In this research, we first found out the formation of nanoscale twin crystals in polymers under cylindrical confinement. The nylon-6 nanorods are fabricated by infiltrating the melt into cylindrical nanopores of an anodized alumina oxide template. The alpha-phase nanotwin of nylon-6 under cylindrical confinement has been examined by both two-dimensional wide-angle X-ray diffraction and transmission electron microscopy (TEM) selected-area electron diffraction (SAED) techniques. The 45 degrees angle between two 200 reflections in the TEM SAED pattern of individual nylon-6 nanorods precisely confirms that nanocrystals in the nylon-6 rod are twins. The twin axis is along the a-axis orientation, which is also the hydrogen-bond direction of the unit cell. The (001) composition (twin) plane is the hydrogen-bonded sheet.