Abstract: Thermoelectric materials can realize the direct conversion of heat and electricity by using the internal carrier transport characteristics of solid. This process is noiseless and pollution-free, which has a wide and impor-tant potential in application. Thermoelectric conversion efficiency depends on the material itself. Continuously improving the performance of existing thermoelectric materials and developing new high-performance thermoelectric material systems are important research hotpots in the thermoelectric field. The type, content and microstructure control of the second phase are the keys to design high-performance thermoelectric composite materials. Compositing the second phase is an effective strategy to optimize the thermoelectric performance, which can not only reduce the lattice thermal conductivity by introducing phonon scattering centers, but also improve the Seebeck coefficient through the energy filtering effect. Meanwhile, this strategy enhances the conductivity by the percolation effect, owning to form a conductive network in the matrix. This paper first introduces the common physical effects in composites, and then reviews the recent progress of the research on several typical thermoelectric mater-ials. The effects of the second phase on the electrical and thermal transports will be also discussed.