The diffusion coefficients of probe ethane molecules charged into long-channel cavities in the hexagonal columnar phase of oriented poly(p-biphenylene terephthalate) with long n-dodecyl side chains have been successfully measured in the directions parallel and perpendicular to the long channel axis by means of a pulsed-field-gradient spin-echo (PFGSE) H-1 NMR method, to elucidate the nature of the cylindrical channel cavities. On the basis of these experimental results, the structural characterization of the long-channel cavities has been made. Furthermore, it is shown that ethane molecules are diffusing to neighboring channels through defects in the wall of the channel cavity, in addition to diffusion parallel to the channel axis.