CaO-Al2O3-SiO2 system glasses with MgO were prepared by melt quenching. The glass structure was characterized by Infrared spectra (IR) and Differential Thermal Analysis (DTA). Dielectric properties, devitrification and chemical durability have been studied. IR spectra and DTA indicated that glass network structure change by doping MgO. The glass chemical improved and dielectric constant decreased after doping with MgO. The glass shows a severe tendency of phase separation.