Abstract: Scale effects play an important role in evaluating mechanical properties of jointed/fractured rock masses. These phenomena were studied through use of the distinct element method. The Voronoi diagram was introduced to simulate stochastic joints, and the coefficient of variation (CV) was used to estimate the resolution of a certain property from numerical tests. A large number of numerical tests with different geometric sizes were carried out. Then, the scale effects of equivalent continuum elastic modulus, E_eq and equivalent continuum elastic compliance, 1/E_eq were statistically analyzed. The investigation shows that: the estimated deformation REV size is about 4 times of average side length of a Voronoi polygon with acceptable variations for CV of 5%; the mean value of specimen’s E_eq obviously decreases as size increases, and the mean value of specimen’s 1/E_eq increases slightly with increasing size. In discussing the applicability of the results, it is better to adopt the reciprocal of a mean specimen’s 1/E_eq as the lower bound of E_eq, while the mean specimen’s E_eq should be used as the effective E_eq in engineering. This method was finally applied to analysis deformation data from in situ tests of Baihetan Hydropower Station.