The removal effects of VOC vapors in a pilot-scale biofilter have been studied. Biofiltration test was performed in a biofilter fed downward with contaminated air at ambient conditions. The biofilter was packed with the bark of a tree(1∼2 ㎝ size). The biofilter was operated under various inlet-airstream benzene concentrations. The flow-rate and temperature in the biofilter were 0.398 L/min., 5∼15℃, respectively. We investigated the removal efficiency variation of benzene with different concentrations were changed. Also, we studied the effects of VOC treatment when microorganisms (Pseudomonas) was added to the biofilter. The results showed that the removal efficiency was fairly improved by addition of microorganisms.