Abstract：There exist three definitions of safety factor in slope stability analyses by limit equilibrium methods (LEM). Definition 1 defines the safety factor as the ratio of the shear strength of soil to the shear stress required for equilibrium. Definition 2 defines the margin of safety factor as the factor of the strength reduction，by which the shear strength of the soil would have to be divided to bring the slope into a state of critical equilibrium. Definition 3 defines the safety factor as the ratio of the resistant shear force to the driving shear force along a certain slip surface. This study discusses the relation between Definition 1 and 3. A unified algorithm is given，which is used to locate the critical slip lines corresponding to Definition 3 and Definition 2 respectively. It is also proved while the finite element method is used to analyze slope stability，both the safety factor and the potential slip surface based on Definition 3 are different from those on Definition 2. Some irrational results might be obtained if Definition 3 is used in finite element slope stability analysis.
