The influence of distributor on the uniformity of fluidization and the selection of the aspect ratio at a critical fluidization quality which corresponds to the maximum of the experimental to theoretical bed pressure drop ratio have been studied. Two types of multi-orifice distributors and three bed materials having their particle size in the range 70-161 mum have been investigated in an air fluidized bed at ambient conditions. The distributor to bed pressure drop ratio and the operating velocity for achieving uniform fluidization have been analyzed. The aspect ratio is shown to have a significant role in selecting a distributor and fixing the operating velocity particularly when the bed is shallow. A model equation derived from the first principles is proposed to predict the distributor to bed pressure drop ratio in terms of the critical aspect ratio and compared with the data reported in the literature.