Interplanetary coronal mass ejections (ICMEs) are large-scale solar wind disturbances propagating from the Sun and causing a depression of the galactic-cosmic ray (GCR) intensity known as Forbush decrease (FD). IC- MEs generally contain coherent plasma structures called magnetic clouds (MCs). A unique and powerful data analysis tool allowing for the study of the quasi-3-D configuration of a MC is the Grad-Shafranov (GS) recons - truction. The aim of this work is to investigate the role played by the MC configuration in the formation of a FD. A suited full-orbit test-particle simulation has been developed in order to evaluate FD amplitude and time pro- file produced by the MC obtained with the GS reconstruction. Particle trajectories are computed starting from an isotropic flux outside the MC region. In addition, particle diffusion has been modeled by superimposing a small-angle scattering over the unperturbed charged particle motion at each time step. The model allows us to investigate the MC effect on GCR propagation and to study the energy dependence of the physical processes in - volved, as it provides an estimate of ground-based GCR counts observations at different latitudes. A comparison between model results and both space-based cosmic-ray measurements in L1 and ground-based observations suggests a major role of drifts in producing the FD and a reduced contribution of GCR particle diffusion.