N-Nitrosodiethylamine (NNDEA) is a toxic compound which is affranchised into the medium mostly via industrial wastewater. For the investigation of degradation mechanism of NNDEA, the aqueous solutions of NNDEA were exposed to Advanced Oxidation processes (AOP's) such as gamma-irradiation, ozonation, UV irradiation and UV/Ozone (ozone effect) and UV/H2O2 (hydrogen peroxide effect). Since the structure of NNDEA is small, it is rapidly degraded in all processes except UV-irradiation process but forming acetic acid and formic acid just was seen in gamma and UV/H(2)O(2)processes. In the other processes, NNDEA is transformed directly to nitrite and nitrate which they can form NNDEA again in the medium. The concentration of NNDEA was decreased from 50 to 21 mg L(-1)after 2.20 kGy gamma-irradiation. After 2.85 kGy gamma irradiation, NNDEA was completely removed from the medium. The results showed that the concentration of NNDEA decreased with UV-irradiation time, the concentration of 50 mg L(-1)NNDEA was 42, 30, 24 and 19 mg L(-1)after 10, 20, 40 and 60 min UV-irradiation, respectively. It has been investigated that the UV-irradiation does not have much effect on the degradation of 50 mg L(-1)NNDEA solution and requires long-term irradiation. In this study, gamma-irradiation was chosen as the best method because of the complete degradation effect and the prevention of the reformation of the NNDEA.