Because of its excellent properties, zirconia ceramics has already been widely applied. Its phase transformation affects its properties. The research on the mechanism of its phase transformation is very important to control the phase transformation as well as its properties. The valence electron structure of cubic zirconia, tetragonal zirconia and monoclinic zirconia are calculated with the empirical electron theory in solids and molecules in this paper. The results show that the total numbers of the covalent electron pairs which form their strong bond framework are 3.19184, 3.45528 and 3.79625, respectively. According to the viewpoint of the C-Me segregating theory in solid alloys, it can be deduced that the phase transformation order of zirconia should be liquid phase→cubic phase→tetragonal phase→monoclinic phase. The deduction from valence electron structure is completely in accordance with the experimental results, so the electron theory of phase transformation in alloys can be expanded to ceramics materials.