In order to examine the brittle-ductile transition mechanism, nano-scratch experiment was conducted on (100), (110) and (111) crystal plane by using nano-indenter, and scratch morphology was observed by using atomic force microscopy and scanning electron microscope. The critical load and critical depth of brittle-ductile transition of each crystal plane were obtained by analyzing the depth-scratch distance curve and the scratch morphology. The results showed that single crystal germanium has strong anisotropy, the critical load of brittle-ductile transition on (100), (110) and (111) crystal plane are 37.6 mN, 30.5 mN and 32.4 mN, the critical depths are 594.7 nm, 512.5 nm and 536.6 nm respectively. A most plastic remove and least brittle-ductile transition on (100) crystal plane during the process of nano-scratch is due to its minimum hardness and deepest depth of brittle-ductile transition, and with the increase of speed of scratches, critical depth and critical load of brittle-ductile transition are increased. At last, the correctness of critical load and critical depth of brittle-ductile transition was verified through constant load scratch experiments.