Abstract: Representative elementary volume(REV) is usually adopted to discuss the variation of permeability with volume of fractured rock masses in multi-field coupling analysis.In fact,REV,as a fundamental conception in rock mechanics,is the basis for choosing rock mechanical model and determining rock mechanical parameters.The purpose of this paper was to present a comprehensive description of the REV of fractured rock masses.Based on the knowledge of rock matrix and rock fractures,six scales were defined to describe the size effects encountered in rock mechanics.Three methods,called Energy superposition method(ESM),geological statistic method(GSM) and numerical simulation method(NSM) were proposed to estimate the REV sizes of fractured rock masses.Mechanical model was discussed according to the relationship between REV and rock,and it was suggested that REV should be regarded as a quantitative criterion for choosing equivalent continuum approach or discrete approach.New conceptions of REV-based and 5S-related mechanical parameters and parameter fields were proposed to reflect the structure,size,stress,strain and seepage effects of mechanical properties of fractured rock masses.