Monte Carlo simulation represents nowadays one of the powerful approach for the simulation of very complex environments like those typical of medical physics where, in general, an accurate simulation of the involved radiation beams and of the patients are required to fully reproduce a clinical case. It since from 1963, when Berger introduced the condensed approach for the simulation of electron interaction with matter grew being today one of the most important tools used to verify the dose distribution in patients, design radiotherapy facility, study the radioisotopesproton/ion beams and their application (Berger. Monte Carlo calculation of the penetration and diffusion of fast charged particles, vol. I. Academic Press, New York, pp. 135–215, 1963 ),(Berger and Hubbell. XCOM: photon cross sections on a personal computer, Technical Report NBSIR 87–3597. National Institute of Standards and Technology, Gaithersburg, MD, 1987. ). Monte Carlo is also often used to evaluate important parameters related with the quality of a radiation treatment . Evaluation of radiobiological damage from charged particles represents a complex calculation where a simple analytical approach is not sufficient for a precise and complete description of involved phenomena. In this work we will present, after a brief introduction on Monte Carlo method, the use of the open-source Geant4 toolkit for the simulation of a typical hadrontherapy passive beamline and how it can be efficiently used to retrieve critical parameters like LET and RBE.