Thermoelectric (TE) materials have gained considerable attention for versatile energy applications because they can directly convert waste heat into electric power without environmental pollution. The TE conversion efficiency is determined by the dimensionless figure of merit, ZT (=S2σT/κ, where S, σ, T and κ are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively). Therefore, the simultaneous realization of high power factor (S2σ) and low thermal conductivity is key to the development of highly efficient TE materials. However, the ‘trade-off’ relationships between the TE parameters in terms of carrier concentration have hindered the practical application of TE materials over the decades. In this presentation, I would like to introduce the effective ways for independent control of electron and phonon transports by controlling grain boundary interfaces in TE materials. Both the enhancement in power factor and the reduction in thermal conductivity could be simultaneously achieved thanks to the beneficial effects of interface control, and detailed transport mechanisms in the interface-controlled TE composites will be discussed.