This paper concerns a comparative study between a 3D full-digital model and a 3D semi-digital model with the aim to represent the gravitational placing of granular mixtures with a very high number of particles. Both models use a completely digitized volume and a similar filling procedure considering random local rules for the particles displacements and taking into account only steric repulsion between particles. The full-digital model is based on the digitization of the particles and their displacements. The semi-digital model takes into account the curvature of particles's surface and displacements of particles depend on real numbers instead of integers. Mono-sized spherical particles systems in homothetic conditions were firstly studied with both models. It is shown that the full digital model is very sensitive to the size of particles. Random loose packing of mono-sized systems can be simulated correctly only when the radius of particles is equal or higher than 4 pixels. On the contrary, the semi-digital model allows a good representation of the random loose packing of mono-sized spherical particles systems independently of the particles radius. The comparison between the two models also concerns the study of binary mixtures with varying proportion of small particles. The packing density obtained with the full digital model for mixtures having low radii ratios and low proportions of small particles are higher than expected by the theoretical approach for binary mixtures without interactions. This model has therefore limited possibilities for the simulation of binary mixtures with radii ratios lower than 4/10. The semi-digital model eliminates these drawbacks but remains sensitive to the displacement amplitude of particles during placing. However, the placing of binary mixtures with this model is coherent with the behaviour of real granular mixtures for radii ratios as low as 1/10. (C) 2004 Elsevier B.V. All rights reserved.