We predict a large thermoelectric figure-of-merit in Kondo insulator nanowires at low temperatures. The high ZT values are due to the Kondo effect for electrons and boundary scattering on phonons. We simulated the electron properties of the bulk Kondo insulators within the framework of dynamical mean field theory and found that electrons have short mean free path. In nanowire structures, electron transport is hardly affected by the boundary scattering due to their small intrinsic mean free paths while phonons are strongly scattered due to classical size effect. The results suggest that the nanostructures of Kondo insulators can be designed for high performance thermoelectric cooling devices at low temperatures.