The ZrCuSiAs structure type compounds represent ultra low lattice thermal conductivity, which may results in a fantastic thermoelectric performance. Here a systematic investigation on the thermoelectric performances of ZrCuSiAs structure type materials LaZnPnO (Pn = P, As and Sb) by first principle calculation is performed. The outcomes represent that the LaZnPnO have ultra low lattice thermal conductivity, which could due to the layered structure and the presence of heavy atoms. The figure of merit of LaZnSbO are the highest among compounds LaZnPnO (Pn = P, As and Sb), varying from 0.29 to 0.79 around the temperature extent begins with 300 K ends in 800 K, however, a utmost ZT of 0.88 is obtained at 900 K in material LaZnAsO, because of its higher electrical conductivity and Seebeck coefficient than those of LaZnSbO in high temperature. The great ZT of LaZnPnO with extreme low lattice thermal conductivity indicates a promising application in thermoelectric devices.