The hardness and elastic modulus of ultra thin amorphous carbon overcoat (COC) films were measured using a recently developed sub-nm nanoindentation system. The carbon overcoat film thickness was varied to be 2.5 nm, 5 nm, and 10 nm on a glass substrate with a 2 nm titanium interlayer. A very sharp indenting tip, which was a cube corner tip with a radius of 44 nm, was used for the experiments. It was found that the mechanical properties of sub-10 nm film thicknesses can be reliably measured using the sub-nm indentation system and a sharp indenting tip. As the thickness of the carbon overcoat increased, so too did the surface roughness. For all three film thickness samples, the trends of hardness and elastic modulus values with the contact depth are very similar. When the contact depth is smaller than the film thickness, the measured values of hardness and elastic modulus are higher than those of the glass substrate, and gradually decrease and then approach the values of glass substrate. When the contact depth is larger than the film thickness, the measured values approximate those of the glass substrate. The thinner film shows higher values of hardness and elastic modulus near the surface, which indicates that mechanical properties do change with film thickness and that measurements made on thicker films and extrapolated to thinner films may lead to incorrect conclusions.