The pop-in behavior and mechanical properties of sapphire crystal vertically indented to its rhombohedral R (10 (1) over bar2) 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 h < 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 (H = 27.5 GPa), true hardness (H-true = 68.9 GPa) at the non-ISE region and effective indentation modulus (M = 423 GPa) for the contact depth of h(c) > 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 (10 (1) over bar2) plane, was estimated as {10 (1) over bar1} <(1) over bar2 (1) over bar0 >. The distributions of corresponding resolved shear stress and principal stresses at the slip plane were also estimated.