To obtain insight into the role of biological activated carbon (BAC) in the operation of the denitrification process using external carbon source, the effect of C/N ratio on bioregeneration of BAC was investigated using a fluidized bed reactor. The C/N ratio was controlled by the semi-continuous mode in which NO3-N was continuously supplied to each column. The supply of sucrose as a sole organic carbon was controlled during the experimental period. The BAC under steady state of the continuous experiments was supplied to the semi-continuous experiment. The organic carbon was supplied to each column for 20 minutes in all semi-batch experiments, but the intervals were varied from 8 to 24 hours. The influent organic carbon concentrations were controlled to maintain the average C/N ratio at 7.06 for all semi-batch experiments. In the semi-continuous experiment, the five columns have achieved an average denitrification efficiency ranging from 71% to 99%, though the influent C/N ratios were varied from 0 to 508. This is due to the adsorption and desorption capacities of BAC. BAC plays a role as a storage tank for the residual sucrose using its micropore during the time when sucrose was supplied, and BAC supplied the sucrose to the denitrifying bacteria as a carbon source so as to keep high nitrogen removal rate when sucrose was not fed to the columns. These results suggest that bioregeneration may be influenced by the C/N ratio in the environment around BAC.