The internal mechanism underlying the high surface activity of t-YSZ nanomaterials composed of low Miller index facets is not completely understood at present. Using first-principles calculations, the surface energies, as well as geometrical and electronic properties of t-YSZ morphology are simulated and analyzed by conventional bulk surface and new microfacet models. The results show that the surface energies follow the trend E-surface [(0 1 0 x 0 1 0)] > E-surface [(1 1 1 X 0 1 0)] > E-surface [( 1 0 1 X 0 1 0)] > E-surface [( 1 1 1 X 1 0 1)] > E-surface [(1 0 1 x 1 0 1)] > (1 1 1) bulk surface > (1 0 1) bulk surface > (0 1 0) bulk surface, where the surface energies of the microfacet models are several times greater than those of the bulk surfaces. The surface activity of the microfacet is therefore much more vigorous than that of the bulk surface. The very high chemical activity of the microfacets is derived from their large Fermi energy, pseudo energy gap, and change in Mulliken population. Although the low Miller index surfaces exhibit weak activity, their interface, such as [(0 1 0 x 0 1 0)], have high chemical activity, because of which they are easily and quickly corroded by CaO-MgO-Al2O3-SiO2 (CMAS), as confirmed by experimental reports. Hence, our findings can be considered a first and vital step toward understanding the unusual properties of nano-YSZ.