Using X-ray scattering complemented by microscopic methods, we studied the alignment of a semicrystalline polymer-isotactic polystyrene-by crystallization in a high magnetic field (7 and 11 T). As reported previously, strong orientation of crystalline chain segments perpendicular to the magnetic field can be achieved. We present a method to determine the degree of orientation from the wide-angle X-ray scattering pattern in a quantitative way. The degree of orientation strongly depends on the crystallization temperature, allowing for nearly perfect orientation only at low supercooling. Furthermore, it is strongly influenced by the number of nuclei initiating spherulitic crystalline growth. Only for high nucleation density, such as that achieved by self-seeding, is strong macroscopic orientation obtained. Microscopic inspection of samples with a low number of nuclei shows that the magnetic field orients only the central part of large spherulites and the orientation is lost in the outer part of the spherulites; alignment takes place only in the initial stage of growth of a spherulite. The strong effect of the temperature of crystallization indicates a competition between orientation and growth.