Nano-indentation of glassy carbons (GCs) heat-treated at different temperatures was carried out using spherical tipped indenters with different radii. Analysis of the force-displacement data enabled indentation stress-strain curves to be generated. It was found that the elastic modulus and the yield stress of GCs decreased with increasing the heat treatment temperature. It was also observed that the radius of spherical tipped indenters depended on the yield stress. From a simple the power law fitting of a non-linear elasto-plastic (contact stress-strain) deformation, beyond the initial elastic deformation, a better measure of hardness for brittle materials is proposed.