Field survey and laboratory experiments were conducted to clarify the effect of secondary effluent from wastewater treatment plant on nitrogenous BOD (N-BOD) behavior in the river.The results showed that the average contribution ratio of N-BOD to total BOD in upstream side where secondary effluent was not discharged, was 9.3%. Whereas the ratio raised to 42.3% in downstream side. It was observed that water temperature and NH4-N concentration were associated with N-BOD. The experiments through the acclimation of biofilm on the river-bed showed that the time lag of nitrification in BOD test during 10-25°C was 1-3 days if C/N ratio was 1. On the other hand, if C/N ratio was more than 5, N-BOD was not recognized at 25°C. The correlation between the observed N-BOD and the calculated N-BOD were obtained (γ=0.89). The values of calculated N-BOD were about 1.3 times larger than those of observed N-BOD. The water temperature and C/N ratio for nitrifying bacteria on the river-bed were found to be important factors to affect N-BOD in the river.