In order to study the mechanical properties of single crystal germanium at micro-nano scale, nanoindentation experiment was conducted on the Ge (100) (110) and (111) by using nanoindenter. Atomic force microscopy was applied to observe the surface of the material. Furthermore, the relationships between the elastic recovery rate, hardness, elastic modulus and indentation depth of germanium was analyzed according to the displacement-load curves of Ge(100) (110) and (111). Results show that elastic deformation, plastic deformation and brittle fracture occur in the loading process of single crystal germanium. When the indentation depth reaches 500 nm, there are pop-in points on the loading curve, while the pop-out points show up on the unloading curve when the indentation depth reaches 100 nm. Also, the pile-up behavior occurs in the residual indentation morphology of single crystal germanium, which demonstrate that single crystal germanium has a low tendency to work hardening. When the indentation depth reaches 100 nm, single crystal germanium shows an obvious size effect, and Ge(111)has the lowest hardness and elastic modulus, which shows that Ge (111) has the best plastic deformation ability relative to the other two crystal planes.