Simulation of a 0.1-µm MOSFET's characteristics using the Monte Carlo method is introduced in this paper. The studied device is a 0.1-µm MOSFET on an ultrathin nearly intrinsic SOI structure that is thought to be useful to suppress short-channel effects. To carry out the calculation, intravalley scattering with acoustic phonon and intervalley inelastic scattering have been taken into account in our model. Surface roughness scattering has also been considered in a particle manner using a classical model, which is a combination of both specular reflection and diffused scattering. In order to take the avalanche breakdown phenomena into account, a two-carrier many-particle Monte Carlo method has been used here. We proposed a new model for the impact ionization probability, and also for the velocity distribution of both the primary electron and the generated electron-hole pairs in this paper.