A number of experiments were conducted in order to investigate the organic removal efficiency and biomass characteristics according to the organic shock loading rate in a fluidized bed biofilm reactor. At the operation conditions of HRT, 8.44 hour, superficial upflow velocity, 0.9 ㎝/sec and temperature, 22±1 ℃, the removal efficiency of SCOD was founded to be 96.5, 92 and 90 % with the organic shock loading rate of 3.5, 10.8 and 33 kgCOD/㎥·day, respectively. Within the F/M ratio ranged 0.4 to 2.0 ㎏COD/㎏VSS·day, the SCOD removal efficiency was shown as 90% at F/M ratio of 2.0 ㎏COD/㎏VSS·day, but the TCOD removal efficiency was 72 % at F/M ratio of 1.8 kgCOD/kgVSS·day. The average biomass concentrations were 7800, 14950 and 27532 ㎎/l on the organic shock loading rate of 3.5, 10.8 and 33 ㎏COD/㎥·day, respectively. This result was agreed with the fact that more biomass could be produced at high concentration of substrate, but some biomass was detached at the onset of shock and easily acclimated at the shock condition.