High dielectric constant (k) CaCu3Ti4O12 (CCTO) has been studied as a potential filler for low-temperature cofired ceramics (LTCC) based on typical calcium aluminoborosilicate glass. CCTO does not appear to be chemically reactive with the glass regardless of the firing temperature over the compositional range of 30-90 wt% CCTO. Dielectric constant and dielectric loss turned out to depend strongly on the relative content of CCTO and firing temperature. As an optimal composition, the glass with 60 wt% CCTO exhibited a k similar to 32 and tan delta similar to 0.01 as a result of firing at 850 degrees C, which is preferably applicable for the medium k LTCC applications. The higher content of 90 wt% CCTO needed to be densified at 900 degrees C for better densification with a higher dielectric constant of similar to 150. However, the 900 degrees C firing was not acceptable for the 30 and 60 wt% CCTO-containing samples due to the unexpected expansion of samples leading to significantly enlarged pores.