In this study, a 34.5CaO–23B2O3–33.5SiO2–6Al2O3–3ZnO (CBSAZ) glass-ceramic substrate was prepared using a sol–gel method. A porous CaO–B2O3–SiO2–Al2O3–ZnO layer (CBSAZ0.5C) with a porosity of 72.6 % and thickness of 365 μm was inserted between two ∼120 μm thick dense CBSAZ layers to form a tri-layer substrate (CBSAZ/CBSAZ0.5C/CBSAZ). The dielectric constants (εr) and dielectric losses (tanδ) of the dense CBSAZ substrate at 20 and 60 GHz were 6.90 and 0.029, respectively, and 6.96 and 0.017, respectively. These values were almost independent of frequency. In contrast, the εr and tanδ values of the CBSAZ/CBSAZ0.5C/CBSAZ sandwich substrate at 20 and 60 GHz were 3.59 and 0.022, respectively, and 3.11 and 0.020, respectively. Thus, the εr value of the CBSAZ/CBSAZ0.5C/CBSAZ sandwich substrate was significantly lower than that of the CBSAZ substrate, whereas the tanδ and κ values of the CBSAZ/CBSAZ0.5C/CBSAZ and CBSAZ substrates were similar.