Influences of size segregation of fluidized particles and the feeding height of an object on its floating and sinking in a gas-solid fluidized bed were investigated. Unclassified sands were used as fluidized particles in order to reduce cost and skip classification process in fluidized bed medium separation. When superficial air velocity ratio was relatively small, specific gravity at the top and bottom of fluidized bed differed because of particle size segregation. Furthermore, the floating and sinking varied according to the feeding height. As a result, the separation accuracy deteriorated. On the other hand, when superficial air velocity ratio was relatively large, the floating and sinking did not vary with the feeding height. However, the floating and sinking were unstable, and the separation accuracy also deteriorated because fluidization intensity was too strong. Hence, when unclassified sands are employed as fluidized particles, intermediate superficial air velocity ratio should be selected in order to prevent particle size segregation and strong fluidization intensity.