Very high power factor of "'131 μW K-2 cm-1 have been achieved on nanostructured YbAgCu4, which is much higher than other nanostructured thermoelectric materials. The grain boundary scattering of phonons reduces the lattice part of thermal conductivity and ZT of "'0.11 have been achieved at 42 K. Such a high ZT at this low temperature is rare and this result demostrates the potential application of this materials for cryogenic cooling.