The thermal properties of any material are as important as mechanical properties due to the fact that mechanical strength degrades with thermal fatigue. In this chapter, the effect of different types of ceramic micro- and nanofillers on the glass transition temperature, thermal stability and decomposition, curing behavior, thermomechanical properties, thermal expansion coefficient, and thermal conductivity of polymer/ceramic composites have been introduced. The involved mechanisms of improvement or degradation are discussed. The effect of particle size, surface-treatment, hybridation, filler ratios, and filler morphology on the synergistic improvement of the thermal performance of such composites are presented and compared in detail.