Crystal structure of ethylene (E) tetrafluoroethylene (TFE) alternating copolymer of TFE 50 mol 96 content has been determined on the basis of 2-dimensional X-ray fiber diagram measured for the uniaxially oriented sample. Different from the unit cell parameters reported previously by several researchers, the newly proposed unit cell is of triclinic type with a = 8.46 angstrom, b = 5.67 angstrom, c (fiber axis) = 5.00 angstrom, alpha = 83.0 degrees, beta = 97.0 degrees, and gamma = 89.7 degrees and the space group Pi. Two slightly deflected but essentially planar-zigzag chains are packed in the unit cell with the zigzag planes parallel to the a + b direction. These two chains are connected by a point of symmetry and are packed upward and downward along the chain axis. In contrast to the relatively sharp X-ray equatorial line profile, the first and second layer line profiles are diffuse. At the same time a series of observed meridional 00 reflections are found to show relative intensity different from the regular chain packing structure. These hkl and 00l reflection profiles were found to be quantitatively reproduced by assuming an aggregation of regular domains with the c-axial translational disorder between them.