The crystallographic orientation of α-Al2O3 powder compact under a magnetic field is affected by the presence of aggregates. In this work, the influence of aggregates on the orientation degree was studied from the experimental and theoretical viewpoints. The optical parameter of birefringence was used as an evaluation index for the crystallographic orientation of α-Al2O3 compact samples. The effect of aggregates on the orientation degree was estimated from the simulation of the distribution of the relative angles between the crystal axes of primary particles for any constituting number of aggregates. The orientation of an aggregate depends on the apparent crystal axes calculated from the vector summation of the crystal axes for all constituting particles. The measured degree of orientation of the powder compact was consistent with the theoretical values for all powder compact samples containing various concentrations and constituting numbers of aggregates. The elimination of aggregates is the key for good particle alignment under a magnetic field.