Abstract: An experimental study of the influence of pressure on Umf and bed expansion height by the photographing with a camera was carried out in a fluidized bed of 60mm i.d.in the range of operating absolute pressure 0.1MPa~1.5MPa with two kinds of different size particles . Experimental results indicate that for the polystyrene(Ps) particle with a larger size， Umf changes with p-0.3, and for the sand particle with a smaller size， Umf changes with p-0.21. An empirical equation about Umf is proposed based on the experimental data: Umf=μdp ρg｛［(34.15)2+0.05916×dp3 ρg( ρs- ρg)gμ2］12-34.15｝ It is found that the bed expansion height increases with the increasing of gas velocity, and increases with pressure for fixed value of UUmf, when the pressure exceeds 0.7MPa, the influence of pressure on the bed expansion height is weakened. For the polystyrene(Ps) particle, UUmf decreases with the increasing of pressure for fixed value of H/Hmf . When H/Hmf＝1.4～1.6，UUmf varies with p-0.52～p-0.58.