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Single-crystal sapphire is utilized as a high-performance engineering material, especially in extreme and harsh environments. However, due to its extreme hardness and brittleness, the machinability of sapphire is still a chal-lenge. By means of nanoindentation and plunge-cut experiments, the anisotropic brittle-ductile transition of the prismatic M-plane and rhombohedral R-plane is examined by analyzing crack morphologies and the critical depth-of-cut (CDC). The experimental results of the nanoindentation tests are correlated to the plunge-cut ex-periment. Both the prism plane and the rhombohedral crystal plane exhibit a two-fold symmetry of ductility with various crack patts along the machined grooves. The direction-dependent plasticity of the hexagonal sapphire crystal is mainly connected to a twinning process accompanied by slip dislocation.