YSZ hollow microspheres are synthesized through an efficient microwave-assisted hydrothermal method. By analyzing products collected at different heating stages, the formation mechanism of the hierarchically porous architecture is proposed. During the low-temperature microwave-heating step, the surfactant self-assembles into spherical micelles. Meanwhile, the Zr4+ and Y3+ ions in solution precipitate onto the template of the as-formed CTAB micelles. During the following hydrothermal process, ethanol dehydrates to form ether in the form of oil droplets in solution. Further aggregation of obtained spherical particles in the microwave-heating step occurs at the oil droplet-solution interface while aging. The resulting YSZ hollow spheres not only contain three levels of hierarchical porosity but also exhibit high BET surface area (163 m(2) g(-1)) and pore volume (0.19 cm(-3) g(-1)). Besides, ultralow thermal conductivity of 0.030 W m(-1) K-1 is achieved at room temperature in such hierarchically porous YSZ hollow spheres. (C) 2014 Elsevier Ltd. All rights reserved.