Numerical simulation of tribo-electrification of particles in a gas-solids two-phase flow has been conducted. Under any injected conditions on the particle, normal component of impact velocity and number of impacts of a particle per unit length have, respectively, a constant value for the calculation beyond the particle stopping distance and impact charge increases linearly with the distance. Although gravity force changes particle motion dramatically, the impact charge increases about 10%. We have also investigated the effects of particle size, air velocity and pipe diameter on the particle motion and the impact charge, which revealed that both the normal component of impact velocity and the number of impacts of a particle decrease exponentially with increasing Stokes number. However, it was found that the effects of air velocity and pipe diameter on the impact charge are qualitatively different from those of the experimental results.