The purpose of this study is to clarify the relationship between mechanical stability of rock block and its vibration characteristics for arbitrary shape and material properties of rock block so as to propose a new method of rock stability evaluation by measuring the vibration of rocks using a remote vibration measuring system. To detect unstable rock masses that are at risk of falling onto railroads, research on safe, efficient, and quantitative evaluation of such risk using a laser Doppler velocimeter (U-Doppler) capable of contactless remote measurement is conducted. In the study, block models representing rocks with different geometries, masses and material properties were used to assess the effects of rock shapes and scales based on the relationship between the dominant frequency and mechanical stability of the model through 2-D FEM analysis. The result of the analysis shows that the relationship between the dominant frequency and mechanical stability is affected by aspect ratio of rock block and dominant frequency reaches a peak when the aspect ratio is around 0.6-0.7 for same stability of the rock block. To further verify the applicability of this block-model simulation, another model representing actual rock-mass geometry randomly defined from 2D-profiled spectra for on-site rocks was included in the analysis. Using this model, the relationship between the dominant frequency and mechanical stability was simulated to determine its acceptability based on comparison of the results.