Abstract: Anaerobic oxidation of ammonium (Anammox) is a newly discovered pathway of nitrogen cycling in natural ecosystems. Recently, it has been found that paddy soils can release N by anaerobic ammonia oxidation, however, few studies on determination in paddy soils of anaerobic ammonia oxidation activity have been published. Based on different mechanisms, isotopic-tracing technique was a new method which can quantitative analysis gaseous nitrogen produced both by denitrification and anammox simultaneously. In this study, we conducted an isotope-tracing technique for determination of anammox in paddy soils. After substrate consumption of nitrate and nitrite, three treatments including 1) 15N-NH4+only as negative control, 2) 15N-NH4+ and 14N-NO3- as positive control, and 3) 15N-NO3-only, were incubated under anaerobic condition.The gaseous N2 was detected with isotope mass spectrometer for separation of 28N2, 29N2 and 30N2. Then the rates of anammox activities were calculated. The established method may provide reliability of anammox activity in paddy soils in situ.