The crystal structures and phase transformation of zirconia ceramics have been investigated by means of X-ray powder diffraction and Rietveld powder diffraction profile fitting technique. A structural transition from monoclinic to tetragonal occurs when Y2O3 and2 are doped into zirconia. The space group of the tetragonal structure is P42/nmc,Z=2. The lattice parameters are α=0.362 6(5)nm,c=0.522 6(3)nm for CeO2 doped zirconia and α=0.360 2(8)nm,=0.517 9(1)nm for Y2O3 doped zirconia, respectively. In each unit cell, there are two kinds of equivalent positions, i.e. 2b and 4d, which are occupied by Zr4+, M(M=Y3+, Ce4+) cations and O2- anions, respectively. The crystallographic correlation among the cubic, tetragonal and monoclinic structures of ZrO2 is discussed.