Diffusion profiles of Ca in highly densified polycrystalline yttria-stabilized (10 mol %) ZrO2 were investigated by secondary ion mass spectroscopy (SIMS). Lattice diffusion coefficient in temperature range of 1073 to 1273 K was determined : D-Lattice = 0.458 exp[-393.3 (kJ/mol)/RT] (cm(2)/sec). Grain boundary diffusion was estimated based on the Whipple＇s analysis. As a result, values of D(Boundary)delta s can be represented by D(Boundary)delta s = 4.85 X 10(-1)3 exp[-185.8 (kJ/mol)/RT] (cm(3)/sec), where D-Boundary is the grain boundary diffusion coefficient, delta the grain boundary width and s the segregation factor.