Continuous segregation of particles with size distribution was studied in a particle withdrawal boot attached to the bottom of a fluidized bed which was installed to simulate the particle dynamics in a particle growing reactor. In order to withdraw continuously and preferentially large particles in steady state from the main fluidized bed the boot was mildly fluidized between the incipient fluidization velocities of the largest and smallest size fractions. It was observed that a high degree of segregation can be achieved in the limited range of the fluidization condition at the boot as well as of the particle size and distribution mode at the main fluidized bed. In the acceptable operation range the segregation efficiency of large particles could fairly well be related to those operational parameters for a given boot geometry, and the empirical relationship proposed in this study were in good agreement with experimental observations.