Particle roundness is one of the key factors that influence the mechanical behavior of granular material, however, most existing roundness evaluation approaches are based on visual comparison, those methods always rely on the operator’s experience. In this study, based on 3D ballast model and definition about ‘particle corner’ and ‘particle roundness’ proposed by wadell H, a three-dimensional roundness index was proposed, combine this index with ODEC method, a robust 3D particle roundness evaluation method was included. The method is then used to evaluate the roundness of ballast particles, the result is consistent with former studies, it also shows that the method is well suits for 3D particle models with high resolution and more accurate and objective than other existing approaches when evaluating a single particle model.