The use of uniaxial anisotropic materials in rectangular dielectric resonator antennas (DRAs) increases the radiation from their side walls compared to their top walls due to the fundamental radiating mode, which leads to the improvement of boresight directivity. The different boundary conditions on the walls of DRA are also investigated. The main phenomenon of gain enhancement is also theoretically explained. The proposed method is validated by comparing the simulation results for the cases of isotropic DRAs and anisotropic DRAs (ADRAs). The measured results for an aperture-coupled uniaxial ADRA show an impedance bandwidth of 20.65% between 3.17 and 3.9 GHz and a peak broadside gain of 8.4 dB.