In the search for more temperature-stable materials, herein, we report on the microwave dielectric properties and trends of the rare-earth niobate series (RENbO4), with the main emphasis being placed on LaNbO4. LaNbO4 has excellent microwave dielectric properties, especially an anomalous small positive temperature coefficient of resonant frequency of 9 ppm/degrees C. We present the phase transformation and ferroelastic domain configuration of RENbO4 studied using in situ X-ray diffraction and high-resolution transmission electron microscopy. The structural distortion caused by the phase transformation is a dominant factor in the relative dielectric constant and temperature coefficient of the resonant frequency in RENbO4.