The effects of tetravalent cations on tetragonal zirconia and cubic zirconia were evaluated by investigating lattice parameter variation. XRD observation was made on 2Y-TZP (2 mol% Y 2 O 3 -stabilized tetragonal zirconia polycrystals) and 8Y-SZ (8 mol% Y 2 O 3 -stabilized zirconia) doped with CeO 2 , SnO 2 and TiO 2 . The contents of the tetravalent dopants increased within the solubility limits of the dopants in 2Y-TZP and 8Y-SZ. The ionic radii of the Ti 4 + , Sn 4 + , Zr 4 + and Ce 4 + for eight-fold coordination with oxygen ions are 0.74, 0.81, 0.84 and 0.97 A, respectively. The unit cell volumes of cubic zirconias are in proportion to the ionic radii of dopant cations but the unit cell volumes of tetragonal zirconias are not.