The Ba0.6Sr0.4TiO3 (BSTO) ceramics were prepared by a traditional electrical ceramic method. Effects of co-doping low dielectric constant oxides Al2O3 and MgO on microstructure and dielectric properties of Ba0.6Sr0.4TiO3 ceramics were investigated. The results show that BSTO/MgO/Al2O3 (BMA) ceramics can be sintered at 1300°C. The main crystal phases are cubic perovskite structure Ba0.6Sr0.4TiO3 and face-centered-cubic spinel structure MgAl2O4. Al3+ is easier to enter Ba0.6Sr0.4TiO3 lattice in the replacement of Ti4+ than Mg2+. When Al2O3 content exceeds 20wt%, Al2O3 would react with BaAl2O4 to generate a new phase of BaAl12O19. Proper amount of Al2O3-MgO can reduce the dielectric constant of BSTO ceramics and result in the higher tunability. There is obvious frequency dispersion characteristic in BMA ceramics and the curie temperature of ceramics decreases with the increase of Al2O3. The BMA ceramics with 20 wt% Al2O3 exhibit good dielectric properties, with εr of 1314, tanδ of 0.003 at 10 kHz, and the dielectric constant tunability of 35.2% under a dc electric field of 1.0 kV/mm, which are useful for potential tunable device applications.