This letter reports a simulation study of depth sensing nanoindentation on a rough surface. The estimated mean hardness and scatter are both influenced by roughness, irrespective of whether there is a genuine variation in mechanical property with deformation volume/depth. The scatter due to roughness always decreases with penetration depth. The roughness dependency of hardness is the product of two terms. The first term is independent of property variation with deformation volume/depth and arises due to the discrete nature of multiple contacts during indentation. The second term is due to property variation with deformation volume/depth, as changing roughness changes the aggregate strength of each contact island. ©1997 American Institute of Physics.