Porous yttria-stabilized zirconia (ZrO2-8 mol% Y2O3, YSZ) ceramics with ultra-low thermal conductivity (as low as 0.06 W/mK) could be fabricated by tert-butyl alcohol (TBA)-based gel-casting process with low solid loadings of 10 and 15 vol%. High porosity (52-76%) and fine pores with average pore size of 0.7-1.8 mu m formed after sintering at 1350-1550 A degrees C. These air-containing pores were believed to affect the through-thickness heat transfer propagation, resulting in low thermal conductivity. The thermal conductivity of porous YSZ ceramics with different porosities fits well with computed values derived from Effective Medium Theory (EMT).