Abstract: In discrete element method (DEM) simulation of rock-fill dam compaction, the sizes of the work area and the roller are usually reduced for lower computational cost, and applying a load on the roller equivalent to its real value is the premise of numerical simulation. This equivalent load depends on the roller size, and hence an accurate calculation of it is crucial to simulation accuracy. This paper develops a method of determining the equivalent load on downsized rollers considering the same porosity of dam material to be compacted. First, we use the particle flow code (PFC) to construct a 2D model and a 3D model that describe the features of rock-fill materials: aggregate gradation, particle shape, contact characteristics, etc., and establish a quantitative relationship between the modeled porosity and the real one. Then, we derive a formula for determining the equivalent load on downsized rollers based on the same compaction ability coefficients, and modify it by matching the modeled porosity with the real one, therefore establishing a relationship of the equivalent roller load versus the roller size. A case study shows that the equivalent load calculated by our method leads to the same compaction effect compared to that of the real roller, thus laying a basis for numerical simulation of downsized roller compaction.