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A case study at the Baihetan dam site was undertaken to obtain a representative volume element (RVE) size. Two-dimensional fracture information in an exploration tunnel was used to generate a large threedimensional fracture network. By dividing the entire modeled rock mass into cubes, the volumetric fracture density (P 32 ) value of each cube was determined. The size effect can be determined by changing the cube size. The RVE was determined using P 32 calculation and statistical tests, including Kolmogorov-Smirnov (KS) and Wilcoxon rank-sum tests. The P 32 value depends on the geometrical parameters of fracture density and size; in this study, this value is called the geometrical RVE. P 32 is dependent on fracture density and size, and cannot appropriately reflect certain data such as fracture dip or dip angle. Therefore, we propose a structural RVE (SRVE) considering fracture dip direction, dip angle, density, and size, which together constitute the available information about the fracture network. Therefore, the SRVE is more applicable for use in solving geological problems than the RVE. In this analysis, the KS and Wilcoxon rank-sum tests were used to determine the SRVE size. Keywords Representative volume element Á Size effect Á Statistical test Á Three-dimensional fracture network Abbreviations K Fisher constant KS Kolmogorov-Smirnov test P 21 Total length of fracture traces per unit area (m/m 2 ) P 32 Fracture area per rock volume (m 2 /m 3 ) RVE Representative volume element SRVE Structural representative volume elementV Cube volume (m 3 ) h Acute angle between the scanline and the line normal to the mean orientation of the joint set (°) N Cube number L Length of the overall rock mass (m) W Width of the overall rock mass (m) H Height of the overall rock mass (m) l Serial number of the cube in the x direction m Serial number of the cube in the y direction n Serial number of the cube in the z direction x 0 X-coordinate of a fracture disc center (m) y 0 Y-coordinate of a fracture disc center (m) z 0 Z-coordinate of a fracture disc center (m) a Dip angle (°) b Dip direction (°) (A, B, C) Normal vector of a fracture plane D Fracture disc diameter (m) D-stat Statistic of Kolmogorov-Smirnov test S i (x i )

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