Collisions among particles in solid-liquid flows in a horizontal pipe were observed with a high speed video camera. The frequency of collisions was obtained as a function of height. It reached a maximum in the vicinity of the surface of a moving bed, and decreased with an increase in height. The averaged value of impact and lift-off angle were approximately zero, and the shapes of frequency profiles of both angles were almost symmetrical. A simulation model of solid-liquid flow taking into account the collisions among particles was established and a numerical simulation was carried out to determine the motion of particles. The simulator gave almost the same particle trajectories, profiles of the particle velocity and solid concentration as the experiments.