Abstract: In this paper, a definition of a representative volume element (RVE) based on the meso-structure of concrete, as well as a method for determining the RVE size of concrete is proposed by means of statistical analyses of the mesoscale structural characteristics of concrete. The voxel concept is employed to calculate the statistical variable of a volume fraction, the average diameter and fineness module of a coarse aggregate. Accordingly, the RVE size of one-grade concrete is quantitatively determined. The effects of an aggregate gradation, the aggregate volume fraction on RVE size are also studied.?The data show that: the RVE size decreases with the increase of the aggregate volume fraction and gradually become stable; the ratio of RVE size to the maximum aggregate diameter for one-grade concrete is larger than that for two-grade concrete. The results have implications in choosing a meso-structure of concrete for the mechanical study at meso level and numerical simulation.