Lattice dynamics ab-initio calculations of the new thermoelectric compound La 4 Sb 3 with anti- Th 4 P 3 are reported. Hybridization of the lower optic mode with the acoustic modes is observed between 5.9 and 7.2 meV. As similar observations were done in other thermoelectric compounds such as skutterudites, it is suggested that this is an important effect in order to achieve low lattice thermal conductivity not only for the compounds La 4 Sb 3 and La 3 Te 4 with Th 4 P 3 structure but also for other thermoelectric compounds, whose structures are not formed by cages.