New microwave dielectric materials based on lanthanum borates (La 2O3-B2O3) systems modified with divalent oxides (CaO, MgO and ZnO) and rare earth oxides (Nd2O 3, Sm2O3 and Gd2O3) have been investigated as new candidates applicable for the low temperature co-fireable system. The low temperature microcircuit technology provides practical and viable merits primarily in that inexpensive low-melting conductor (e.g., Ag and Cu) can be utilized with the subsequent increment of integration density. Several different batches of glass consisting of 60 mol% B 2O3, 20 mol% rare earth oxide (i.e., Nd2O 3, Sm2O3 or Gd2O3) and 20 mol% divalent oxide (i.e., CaO, MgO, or ZnO) were successfully prepared by quenching melts from 1100-1300°C. Various amounts of Al2O 3, TiO2 or BaTiO3 filler were then added to form rigid samples through subsequent firing at 850°C for 30 minute in ambient atmosphere. As a specific example of optimized dielectric characteristics, a high-quality factor of ∼1,090 and a dielectric constant of 8.32 at 17.04 GHz was obtained for the Zn and Labased material system. Understanding of developed crystalline phases during the firing process and their correlations to consequential microwave dielectric properties in the newly-developed material systems are the main purpose of this presentation.