Abstract: Method of limit equilibrium is generally applied to assess stability and to calculate sliding force of landslide, which is based on a series concepts such as equilibrium, limit equilibrium, stability, factor of stability, factor of safety, sliding force, anti-sliding force, residual sliding force etc. In the paper, the authors discussed the physical significance of the concepts conscientiously. In the process deducing the formula of sliding force, the relations between factor of stability and factor of safety, among sliding force,anti-sliding force and residual sliding force are commented. A case of example is calculated by the way of Excel, which could be easily understand by engineers.