The present study investigates the depth-dependent hardness observed in nanoindentation tests. The depth-dependent hardness may be caused by deformation within the bulk of the indented material and/or by deformation at the surface of the indented material. The plastic work done by an applied indentation load is divided into plastic bulk work and plastic surface work. The plastic surface work represents the energy dissipated at the surface during nanoindentation. Consequently, an apparent surface stress is defined as the energy dissipated per unit area of the surface. The plastic surface work is necessary for the deformation of a solid surface to form an impression, which relates to the apparent surface stress and the size and geometry of an indenter tip. Good agreement is found between the first order approximation of the theoretical results and the available data of depth-dependent hardness, indicating that the apparent surface stress may play an important role in the depth-dependent hardness. The apparent surface stresses extracted from the nanoindentation tests at room temperature are much higher than the corresponding surface energies at elevated temperatures.