The effects of Sm2O3 addition on the microstructure and the microwave dielectric properties of BiNbO4 ceramics ha;e been investigated. The phase transition temperature (from orthorhombic to triclinic phases) of BiNbO4 ceramics can be lowered with Sm2O3 addition. It also decreases with increasing Sm2O3 addition. The dielectric constant and the theoretical density are found to increase with increasing sintering temperature, but decrease with increasing Sm2O3 content. The quality factor, Q, is significantly dependent on Sm2O3 substitution, due to many factors, including the microstructure, the phase transition, and the densification. The temperature coefficient of resonant frequency, tau (f), is correlated to the phase transition. Bi0.95Sm0.05NbO4 ceramics, sintered at 950 degreesC, exhibits the microwave dielectric properties epsilon (r) similar to 44.43. Q X f values similar to 1800 (at 7.2 GHz), and tau (f) similar to -4.2 ppm/degreesC.