A series of shaking table model tests using two different size shaking tables was conducted to develop a procedure to evaluate seismic slope stability. It was found from the model tests that the type of slope failure could be categorized into two groups. One is the sliding failure, which could trigger the catastrophic failure of the slope and the other is the progressive deformation mode in which the displacement of the sliding mass of the slope would increase gradually. Based on the model test results, it was attempted in this study to propose an index to evaluate the degree of seismic slope instability. In this study, the normalized sliding displacement was applied as an index to assess the slope instability based on the observation from the shaking table model test. Based on the investigation into the correlation between the failure processes of the model slope and the values of normalized sliding displacement, it was found that the proposed index could be used to evaluate the degree of slope instability by setting the proper threshold value.