The authors found that fault displacement (D) linearly relates to distance to tip (L_d) which denotes the distance from a certain offset spot to the fault tip, based on the statistics of 40 sets of (L_d, D) out of 24 faults in Northwest Yunnan. At the same time, according to the relationship of fault crush belt width (W) to fault displacement, the fault length (L) might be obtained by maximum width of fault crush belt (W_max). In this paper, the authors applied the relations mentioned above to quantitatively evaluating the extension toward south of Dingxiling fault as well as the length of Jianchuan fault in Northwest Yunnan.