The applicability and the reliability of the traditional power-law proposed by Oliver and Pharr in describing the nanoindentation unloading behavior of the ion-implanted samples were examined in this paper. Nanoindentation tests were conducted on a tetragonal zirconia polycrystalline (TZP) ceramic implanted by titanium ions with metal vapor vacuum-arc (MEVVA) sources. Both the power law (a paraboloid punch approximation) and the quadratic polynomial (conical indenter approximation) were employed to analyze the unloading portions of the resultant load-displacement curve. It was found that, when the examined contact depth is smaller then the thickness of the ion-implanted layer, the determined power law exponent, m, is much larger than 1.5, the theoretical value for a paraboloid punch. This phenomenon may be attributed mainly to the effect of the residual surface stresses associated with the ion implantation. The hardness and Young's modulus of the ion-implanted sample were found to exhibit a graded distribution and have maxima when the contact depth is comparable to the average projected range of the implanted ions. (C) 2014 Elsevier B.V. All rights reserved.