New ceramic substrates firable at low temperature (<1000$^{circ}C$) were prepared with mixture of alumina and glass powders in CaO-Al2O3-SiO2 system. The substrate of alumina 40 wt% and glass 60 wt%, which was fired between 900∼1000$^{circ}C$, shows low dielectric constant (5∼8 at 1 MHz), specific gravity of 3.10, relatively low thermal expansion coefficient (5.5${ imes}$10-6/$^{circ}C$ at 40∼500$^{circ}C$) and excellent surface roughness (0.4∼0.5 ${mu}$Ra). These properties were thought to be superior to those of conventional Al2O3 ceramic substrates.