The orientation behavior of the three principal crystallographic axes, the a-, b-, and c-axes, was estimated in terms of the orientation distribution function for poly(butylene terephthalate) (PBT) crystallites with a triclinic unit cell. Despite a number of papers reported for the orientation of crytsallites, this paper is the first successful trial for a triclinic unit system. For crystalline polymers with a triclinic crystal unit, there are no crystal planes perpendicular to the a-, b-, and c-axes whose reflection cannot be detected by X-ray diffraction techniques. Accordingly, the functions of the a-, b-, and c-axes must be obtained from the method proposed by Roe and Krigbaum. In doing so, the orientation functions of the reciprocal lattice vectors must be measured by X-ray diffraction for a number of crystal planes. But the measurements are very difficult for crystalline polymers such as poly(ethylene terephthalate) (PET), nylon-66, and PBT with the triclinic crystal unit, since the orientation functions for a number of crystal planes are needed to calculate the orientation distribution function of a-, b-, and c-axes and the measurements are very difficult. Among the three polymers, PBT is the lowest crystallinity, and their diffraction intensities from most of crystal planes are very weak. Nevertheless, as the most difficult trial, the orientation of crystallites and the orientation of the a-, b-, and c-axes were carefully estimated for an uniaxially drawn PBT film in terms of orientation distribution function in order to check the limit of the estimations. As the successful result, it was found that the c-axes oriented affinely with respect to the stretching direction, and the a- and b-axes took a characteristic mode associated with the rotation of crystallites around their own c-axis.