LaYbZr2O7 ceramic thermal barrier coatings (TBC) of meta-stable structure were prepared by an air plasma spraying process. Their microstructure and associated thermal transport properties evolution during high-temperature annealing at 1300 degrees C were characterized. The as-sprayed LaYbZr2O7 TBCs underwent a fast crystallization and a quasi-eutectoid transformation during annealing, resulting in a biphase composite consisting of La-rich pyrochlore phase and Yb2Zr2O7 fluorite phase with coherent phase boundaries. Due to the diffusion barriers between the two phases as well as the low interface energy of the coherent boundaries, sintering and grain growth of materials was significantly refrained. Therefore, a final thermal dynamically stable microstructure with a grain size of 300nm and a total porosity about 5% could be maintained even after long-term aging at a high temperature of 1300 degrees C. Resulting from this stable microstructure, an ultralow thermal conductivity of 1.3W(mK)(-1) could be obtained even after 216h high-temperature aging, which is much lower than that of the state-of-art 7wt% yttria-stabilized zirconia TBCs. Both the high phase and microstructure stability and the extremely low thermal conductivities could be particularly beneficial for TBC material in gas turbine applications.