Superplastic behavior of fine-grained zirconia ceramics (YTZP) is still a major issue on the ceramic science. This work presents a theoretical approach to the deformation mechanism yielding superplastic response in YTZP. In this paper we prove that, in the absence of any relaxation mechanism of internal stresses, a linear dependence of strain rate with stress is obtained, but the invariance of the microstructure cannot be assured. On the other hand, if the grain dynamics is controlled by the relaxation mechanism, a parabolic dependence is expected. This model predicts a transition between parabolic to linear dependence as the grain size increases, in quantitative agreement with the experimental data reported in the literature for YTZP. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.