The polycrystalline ceramics of Sm3Fe5-xAlxO12 (x = 0, 0.5, 1, and 2) are prepared by the sol–gel method. The structure, dielectric, and magnetic properties are studied. The X-ray diffraction patterns indicate that the prepared samples crystallize in garnet phase. All of the samples exhibit relatively high-dielectric permittivity in the low-frequency range. With the increase of Al3+ content, the dielectric permittivity and the temperature and frequency stabilities at low frequency simultaneously decrease, which suggest the important role of Fe3+ for the dielectric property. Two sets of dielectric relaxation are observed in the available temperature from 300 to 700 K. With the increase of Al3+ concentration, the high-frequency relaxation peaks that originate from the carriers hopping between Fe2+ and Fe3+ shift toward the direction of low frequency, and the low-frequency relaxation processes that arise from the doubly ionized oxygen vacancies spread to high frequency. By measuring the hysteresis loops and the magnetization-temperature curves, it indicates that the magnetization is decreased and the magnetic property transforms from ferrimagnetism to paramagnetism with increasing the value of x at room temperature.