Availability of organic media for improving nitrogen removal efficiency in a biofilter process was studied. Although, aerobic and anoxic phase could be clearly divided during the operation of single-stage biofilter process and nitrification efficiency was about 80%, denitrification efficiency was very low due to depletion of organic substrate in anoxic phase. However, the deficiency could be improved by addition of organic media in anoxic phase from the simulated result. Nitrogen removal efficiency in a biofilter process packed with organic media in anoxic phase was predicted to be 29% higher than that packed with inorganic media. In addition, about 79% of nitrogen removal efficiency could be obtained from a column test using organic media innoculated by activated sludge. Improvement in nitrogen removal efficiency was responsible for organic leaching from the organic media and the leaching rate was 0.21㎎COD/g-org.media/day.