The floating and sinking of different density spheres in a gas-solid fluidized bed was investigated by changing the superficial air velocity, bed height, particle diameter and sphere diameter. Vertical shaking of the fluidized bed surface was observed under these experimental conditions to evaluate particle fluidization intensity. Those observations were compared with the results of sphere floating and sinking. Small spheres of diameter = 4.76 mm tended to drift in the fluidized bed without floating or sinking with increasing superficial air velocity. However, the spheres easily floated and sank when the bed height or particle diameter was small even if the superficial air velocity was relatively large. Vertical shaking decreased with decreasing superficial air velocity, bed height and particle diameter. Spheres floated and sank according to the density difference when the vertical shaking was less than I cm. These results indicate that optimum conditions to reduce particle fluidization intensity should be employed to obtain steady floating and sinking of objects in practice.