A wide-angle X-ray diffraction method for measuring poly(trimethylene terephthalate) (PTT) crystal orientation function was described. It was based on Wilchinsky's treatment of uniaxial orientation. Although PTT has a low symmetry triclinic cell only one equatorial reflection is needed for the measurement because the unit cell β-angle happened to be 90°, and there is a strong 010 reflection. The choice of unit cell parameters from the conflicting literature data and the reduction of Wilchinsky's equation into a simple form allowing the measurement of only one reflection were presented. The discrepancies between literature wide-angle X-ray and electron diffraction unit cell volumes and crystal densities were also discussed.