Abstract: By using a thermal-state visualizable fluidized bed,the sticking of iron ore fines with different particle sizes was investigated under the condition of certain superficial gas velocity at 1073 K. It is found that according to different influences of particle size on the sticking,iron ore fines can be divided into three particle size ranges:small particle size at which sticking occurs in the neutral atmosphere under 1073 K,medium particle size at which defluidization happens when iron ore fines are reduced to low metallization ratio on the fluidized bed,and large particle size at which sticking does not appear on the fluidized bed,even to the end of reduction.The influence mechanism of particle size was analyzed in this paper. Moreover,under the normal fluidization condition,the swelling degree of the fixed bed after defluidization decreases with increasing particle size.