It is well known that a small amount of liquid has significant effects on the motion of particles. A numerical simulation of mono-sized spherical particle flows with a small amount of water in a centrifugal tumbling granulator was made by using discrete element method (DEM). A model for the inter-particle force due to a liquid bridge was applied to the present work because granulating processes are usually performed by adding binding liquid. In the present simulation, this behavior was predicted when bridges are formed within the layer. Once the locations of bridge were determined, the water was assumed to distribute uniformly among all locations. In addition to the simulation, measurements were made to validate the simulation results. Particle velocities on the surface of a moving particle bed was measured with a particle tracking velocimetry (PTV) technique. The calculated variation of particle velocities against the water content agreed well with the experiments.