The thermoelectric energy-conversion efficiency is significantly enhanced by introducing magnetism. Doping Cr atoms in a Cu2SnS3(CTS) matrix modifies the electron density of state (e-DOS) and causes a special electron-transport mechanism by localized impurity spin moments. The localized spin moments cause a spin-spin exchange interaction with the spin of itinerant electrons of the CTS matrix. This is clearly verified in M-T measurement, which deviated from Curie's law. The figure of merit (zT) of the magnetic nanocomposite was 8 times higher than that of pristine CTS by the synergetic effect of modifying e-DOS near the Fermi level and electron transport by localized spin moments.