The effect of B2O3 on the sintering temperature and microwave dielectric properties of Ba5Nb4O15 has been investigated using X-ray powder diffraction, scanning electron microscopy, and a network analyzer. Interactions between Ba5Nb4O15 and B2O3 led to formation of second phases, BaNb2O6 and BaB2O4. The addition of B2O3 to Ba5Nb4O15 resulted in lowering the sintering temperature from 1400degrees to 925degreesC. Low-fired Ba5Nb4O15 could be interpreted by measuring changes in the quality factor (Q X f), the relative dielectric constant (epsilon(r)) and the temperature coefficient of resonant frequency (tau(f)) as a function of B2O3 additions. More importantly, the formation of BaNb2O6 provided temperature compensation. The microwave dielectric properties of low-fired Ba5Nb4O15 had good dielectric properties: Q X f = 18700 GHz, epsilon(r) = 39, and tau(f) = 0 ppm/degreesC.