Abstract: Experiment results of the effect of internals in high density circulating fluidized bed (HDCFB) on gas dispersion were reported in this paper .Gas dispersion behavior was investigated by using steady gas tracer (propylene) technique and FCC particles (particle density 1398 kg·m ^-3 , mean diameter 54 μm) for a 0.186 m ID,9.0 m high riser.Experiment results indicated that gas mixing could be well described by a two-dimensional dispersion model. Due to internals, fluid in HDCFB experienced violent roll flowing,and radial distribution of gas concentration was more uniform than in that free bed.The radial dispersion coefficient showed a value of 1000 cm ²·s ^-1 ,a one and a half order of magnitude larger than that in conventional HDCFB.