A new potential low-temperature co-fired ceramics (LTCC) system based on a simple lanthanum borate (La2O3-B2O3) glass was investigated with regard to phase development and microwave dielectric properties as functions of alumina filler content less than 50 wt% and firing temperature up to 1050 degrees C. Unexpected crystalline phases, such as La(BO2)(3), LaAl2.03(B4O10)O-0.54, LaBO3, and Al20B4O36, developed during the firing process are likely responsible for substantial resultant changes in physical and microwave dielectric properties. As a specific example, a high-quality factor of 785 at 15.8 GHz obtained for a composition containing 30 wt% alumina supports the hypothesis that the phase-dielectric property relation exists in this LTCC system. On a practical basis, two phases of LaAl2.03(B4O10)O-0.54 and LaBO3 must be important in determining the final dielectric performance by manipulating the ratios of glass and filler and by selecting a desirable temperature.