The microwave dielectric properties and the microstructures of La2O3-modified BiNbO4 ceramics were investigated. (Bi1-xLax)NbO4 ceramics combined with orthorhombic and triclinic phases are identified at sintering temperatures of 920–980°C. The density and grain size of (Bi1-xLax)NbO4 ceramics both increase with increasing sintering temperature while both decrease slightly with the increase of La2O3 content. The dielectric constant εr, the quality factor Q and the temperature coefficient of resonant frequency τf of the ceramics are affected by the La2O3 content and correlated with its microstructure. At a sintering temperature of 920°C and x value of 0.01, (Bi1-xLax)NbO4 ceramics exhibit the microwave dielectric properties of εr∼43.36, Q×f∼10600 (at 7 GHz), and τf∼-2 ppm/°C.