Granular filters are an essential component in earth dams to protect the dam core from seepage erosion. This paper uses the particle flow method (PFM) to study the mechanism of particle transport in a base soil-filter system. The distributions of the eroded base-soil particles in different filters are traced and analyzed. The eroded mass and intruding depth of the eroded particles into the filters are obtained under different times and hydraulic gradients. The simulation results show that the eroded mass and intruding depth of the base-soil particles into the filter are related to the representative particle size ratio of the base soil to the filter, hydraulic gradient and erosion time. The numerical predictions are also compared with the empirical filter design criterion. The results show that the particle flow model provides an effective approach for studying the filtration micro-property and the erosion mechanism in a base soil-filter system, which is useful for filter design. (C) 2012 Elsevier Ltd. All rights reserved.