The fluidization characteristics of Geldart group A and Geldart group B particles were investigated in four gas?solid micro fluidized beds with different inner diameters ranging from 3 to 20 mm, respectively. At the same time, the variation rules of the minimum fluidization velocity affected by some important factors, such as bed geometry, operating conditions and physical-phase properties were studied. The result showed that the bed pressure drop characteristics in the gas?solid micro fluidized bed were closely related to the used particle type, and the flow characteristics of two types of particles were significantly different under various flow states. In the fixed bed stage, compared with Geldart group B particle, the interaction between the A particles and the wall was stronger, leading to greater deviation of the experimental pressure drop value from the calculated value derived from the pressure formula of traditional fluidized beds. While in the fluidized bed stage, the larger particle size and density of Geldart group B particles showed higher bubble coalescence and rupture degree in the fluidized bed, which intensified the collision among different particles and increased the energy loss, as a result the pressure drop of the whole fluidized beds showed a higher experimental value. The minimum fluidization velocity of the gas?solid micro fluidized bed was not only related to the general operating conditions and gas/solid phase properties but also influenced observably by the inside diameter and the height of the static bed. The minimum fluidization velocity increased gradually with the decreased of bed diameter and increased of the static bed height. An empirical correlation to predict the minimum fluidization velocity of the micro fluidized bed was proposed within the scope of the experimental investigation under the condition of some important influencing factors.