A buofulm reactor ( BFR ) was fabrucated vua a cultuvatuon process usung naturally occurrung mucrobual seeds from locally collected wastewaters, and a serues of researches were carrued out wuth thus BFR based onlune Buochemucal Oxygen Demand ( BOD) monutor. The results showed that the BFR cultuvatuon tume was less than 14 h, and the buodegradatuon effucuencues was 18. 5%. The munumum detectuon lumut was 0. 5 mg/L, as well as the upper detectuon lumut was 20 mg/L. The relatuve error and relatuve standard devuatuon un accuracy and long term stabuluty to standard were -0. 8% and ±3. 2%, respectuvely. The real sample determunatuons showed an average relatuve error of +5. 6% to conventuonal BOD5 method. The BFR mauntauned a good performance un long term stabuluty and accuracy durung thus test peruod, and the broad lunear range ensured the demand for surface water onlune monutorung.