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The pop-in behavior and mechanical properties of sapphire crystal vertically indented to its rhombohedral R (10inline image2) plane were investigated by nanoindentation using a Berkovich indenter. Effect of loading rate on pop-in load and pop-in extension width was observed within the indentation depth of < 120 nm. The indentation size effect (ISE) of hardness within an indentation depth of 60 nm was systematically analyzed using Nix-Gao and Al-Rub models. Our experiments provided the consistent evaluations of hardness (= 27.5 GPa), true hardness (Htrue = 68.9 GPa) at the non-ISE region and effective indentation modulus (= 423 GPa) for the contact depth of hc > 20 nm. Using the Hertzian contact theory, Schmid's law, and energy principle of indentation, the possible dominant slip system, which mainly contributed to the first pop-in event when indented normal to the (10inline image2) plane, was estimated as {10inline image1} <inline image2inline image0>. The distributions of corresponding resolved shear stress and principal stresses at the slip plane were also estimated.