Mixing characteristics of large cylindrical particles such as RDF (Refuse Derived Fuel) were studied in a 0.3 m-I.D. fluidized bed of sand whose diameter is 0.62 mm and density is 2560 kg/m³, respectively. In order to determine the effects of density of cylindrical particles on the mixing characteristics, particles which were made of wood, polyethylene. polyacetal or nylon. The density of particles were varied from 680 to 1510 kg/m³. In order to determine the effects of particle size, the diameter and length of cylindrical particles was varied from 15 to 30 mm and from 20 to 50 mm, respectively. The mixing index increased with gas velocity and decreased with the increase of large particle size. The mixing index shows maximum value with large particle's density at near the minimum fluidized velocity, while at three times the minimum fluidized velocity (U = 3 U_mf), the density of large particle does not live any significant effect to the mixing index. The mixing index was well correlated with Archimedes number and Reynolds number.