Low-temperature co-fired ceramics (LTCC) applied in millimeter/microwave and terahertz frequencies (5G/6G) have attracted a lot of attention recently. In this study, MgO-based dielectric ceramics were successfully sintered at 950°C with the sintering aids: x wt.% of LiF fluoride (x=2, 4, 6, 8, 10) and 0.5 wt.% of BBSZ (Bi₂O₃–B₂O₃–SiO₂–ZnO) glass. BBSZ glass was introduced as another sintering aid to facilitate the sintering and densification. Crystalline structure and micro-morphology were investigated and analyzed. Dielectric properties (ε_r, Q×f, τ_f) at millimeter/microwave and terahertz wave frequencies were also studied. The ionic characteristics of Mg–O bond (f_i), the lattice energy (U) and the bond energy (E) were calculated and analyzed. It is suggested that the optimal x=4, where ε_r=10.5, Q×f=120,000 GHz (@12 GHz) and τ_f=?26 ppm/°C at millimeter/microwave range. When the frequency was up to terahertz (1.0 THz), the ε_r values were 8.8–9.35 and the tanδ were 5.6×10^?3–8.7×10^?3. The experimental results indicated that the low-temperature sintered MgO-based ceramics have potential for millimeter/microwave and terahertz communication applications.