BaCu(B2O5) ceramics were synthesized and their microwave dielectric properties were investigated. BaCu(B2O5) phase was formed at 700 degrees C and melted above 850 degrees C. The BaCu(B2O5) ceramic sintered at 810 degrees C had a dielectric constant (epsilon(r)) of 7.4, a quality factor (Q x f) of 50 000 GHz and a temperature coefficient of resonance frequency (tau(f)) of -32 ppm/degrees C. As the BaCu(B2O5) ceramic had a low melting temperature and good microwave dielectric properties, it can be used as a low-temperature sintering aid for microwave dielectric materials for low temperature co-fired ceramic application. When BaCu(B2O5) was added to the Ba(Zn1/3Nb2/3)O-3 (BZN) ceramic, BZN ceramics were well sintered even at 850 degrees C. BaCu(B2O5) existed as a liquid phase during the sintering and assisted the densification of the BZN ceramic. Good microwave dielectric properties of Q x f=16 000 GHz, epsilon(r)=35, and tau(f)=22.1 ppm/degrees C were obtained for the BZN+6.0 mol% BaCu(B2O5) ceramic sintered at 875 degrees C for 2 h.