New glass–ceramic/ceramic composites based on lead-free low-softening La2O3–B2O3–Al2O3 (LBA) glass–ceramics and Al2O3 fillers were prepared. The phase compositions, microstructure, and dielectric properties of the composites were investigated. LaBO3, La(BO2)3 and LaAl2.03(B4O10)O0.54 including Al2O3 fillers were developed in the LBA glass–ceramics/alumina composites sintered at 875 °C, but the relative amounts of the phases varied greatly with different glass–ceramic compositions and were considered to be determined by the ratio of La2O3 to B2O3. The onset sintering temperatures of green composites were ~ 670 °C and the sintering shrinkage of the samples was mainly determined by the low-softening glass–ceramics. Optimum properties were exhibited for the composite of 25 La2O3–65 B2O3–10 Al2O3 glass–ceramic with 40 wt% Al2O3 sintered at 900 °C with a permittivity εr = 7.91, a dielectric loss tan δ = 2.12 × 10−3 (at 8.1 GHz). The low εr and tan δ coupled with a low sintering temperature, suggested that the LBA glass–ceramic/Al2O3 composites were potential candidates for LTCC applications.