Ba-4(Sm0.15Nd0.85)(9.33)Ti18-zAl3z/4O54 (BSNT-zAl, 0.0 <= z <= 2.5) ceramics were prepared via a solid-state reaction, and the effects of Al doping on the microwave dielectric properties and defect behavior of the title compound were studied. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) photographs suggested that Al ions successfully entered the lattice to form tungsten-bronze-like solid solutions. With a small amount of Al substitution, the relative dielectric constant (epsilon(r)), and the temperature coefficient of resonant frequency (tau(f)) values decreased, whereas the quality factor (Qxf) substantially increased by approximately 50%. The defect-related extrinsic dielectric loss was clarified via the thermally stimulated depolarization current (TSDC) technique. With Al doping, the TSDC relaxation of across-grain-boundary oxygen vacancies (VO..) vanished, whereas that of defect dipoles (AlTi '-VO..) appeared at relatively low temperatures. Therefore, in the BSNT-zAl ceramics, oxygen vacancies were more inclined to interconnect with AlTi ' to form defect dipoles. This could reduce the activity of VO.. and account for the notable improvement in the Qxf values. In particular, the excellent characteristics of epsilon(r)=67.33, Qxf=16530GHz, and tau(f)=+0.87ppm/degrees C were achieved in the specimens with z=1.5 sintered at 1350 degrees C for 4hours.