Newmark sliding block method is a popular engineering method to evaluate permanent displacement of rock slope during seismic loading. In practice, a fixed threshold of horizontal seismic coefficient defined by the critical surface subject to factor of safety equal to 1.0 is used to examine the horizontal component of earthquake. The magnitude of displacement is obtained by integrating twice the difference of the applied acceleration and the critical acceleration with respect to time. However, this procedure does not rationally consider the variation of strength of rock mass during seismic loading and effect of vertical component of earthquake. The direction and quantity of displacement based on this approach is unclear. This study proposed modifications to improve the traditional Newmark sliding block method for consideration of the earthquake in horizontal and vertical directions and the variation of strength of Mohr-Coulomb material during earthquake. The proposed framework, compiled in the EXCEL program, is illustrated with an ideal model of sliding block. The yielding results are compared and discussed in this article. The modified analysis procedure is then applied to a real case of dip slope around reservoir in south of Taiwan for safety of reservoir. Satisfactory results are obtained in the demonstration example analyzed with the proposed framework. The effect of vertical earthquake is also discussed by this case.