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 Y_2O_3 and CeO_2 are doped into zirconia. The space group of the tetragonal structure is P4_2/nmc, Z=2. The lattice parameters are α=0.362 6(5) nm, c=0.522 6(3)nm for CeO_2 doped zirconia and α=0. 360 2(8)nm, c=0. 517 9(1)nm for Y_2O_3 doped zirconia, respectively. In each unit cell, there are two kinds of equivalent positions, i. e. 2b and 4d, which are occupied by Zr~(4+), M(M=Y~(3+), Ce~(4+)) cations and O~(2-) anions, respectively. The crystallographic correlation among the cubic, tetragonal and monoclinic structures of ZrO_2 is discussed.