Hardness characterization is a key issue for the evaluation of the resistance to contact damage of dental materials. In this study, nanoindentation tests were conducted on four commercial zirconia dental ceramics. For each material, the measured nanohardness exhibits significant indentation size effect, i.e., the measured hardness decreases with increasing peak load. The so-called load-independent nanohardness was determined based on a second-order polynomial analysis of the variation of the contact depth with the peak load. Finally, a brief discussion was conducted on the reliability and the applicability of the determined load-independent nanohardness.