The purpose of this study is to clarify the seismic factors and collapse mechanisms of seismic mountain slope failures. The JMA’s seismic intensity (I) and Takemura’s seismic intensity (IT) formulas are used to measure seismic intensity related to slope failures. In order to clarify the relationship between the two, an equation relating the JMA’s seismic intensity (I) to the density of slope failures (Sd), that occurred during the two earthquakes investigated by Ito et al. is presented. Combining this Sd formula with Takemura’s measured seismic intensity formula (IT), we obtain the following formula Sd=ks[av]b where ks, b: coefficients, a: maximum acceleration, v: maximum velocity. This equation implies that the density of seismic slope failure (Sd), has a direct relationship with the Takemura’s tremor factor [av]. Next, taking up the large‐scale collapses presented by Kawabe, six collapses in all, and examining the relationship between the amount of collapse (V) and Takemura’s tremor factor [av] for each collapse, the following relationship V=k[avn]m was shown to hold. where k, n, m: coefficients. Although n=1/2 seems to be the best value for the power factor (n), n=1 is not a low correlation, and this equation may well be consistent with general seismic slope failures, especially surface failures. Furthermore, from the discussion including the energy equation from the formation of the failure layer to the collapse, the following equations ls=fl(vh) and V=fV(avh) were obtained for the slope length ls and the volume of collapse (V) from the consideration from the formation to the collapse of the failure layer. The data analysis shows that the factors in the equations are valid, and the overall agreement is good for the seismic factors a and v. These results suggest that the maximum velocity (v) accelerates the formation of a fracture layer in the interior of the mountain slope, and that the maximum acceleration (a) causes the total collapse of the mountain slope.