Transient behavior of a bed collapsing after cut-off of gas supply into a three-phase fluidized bed was determined in a 0.21 m-diameter half-tube acrylic column having a test section 1.8 m high. The transient behavior of the bed collapse after cut-off of the gas supply to the beds was monitored by a video camera (30 frames/s). A theory was developed to account for the dynamic behavior of the bed collapse after the gas supply shut-off to three-phase fluidized beds. The bubble drift velocity was theoretically calculated by gas and liquid phase holdups at steady state condition. At a liquid velocity of 0.103 m/s and gas velocity of 0-0.023 m/s, bubble size was uniform in the dispersed bubble flow regime. However, as the gas velocity increased above 0.023 m/s, the discrete or coalesced bubble flow regime could be observed. The agreement between the predicted and experimental values is acceptable in the dispersed bubble flow regime, but the agreement becomes poorer with increasing gas velocity.