The combined use of preoxidation treatment and organic polymer flocculant can increase the concentration of nitroso-dimethylamine (NDMA), a nitrogen-containing disinfection by-product. The concentration of NDMA is about 60-100 ng/L under typical water treatment process. In order to understand the influence of different factors and the reaction mechanism, the amounts of NDMA produced after the polyamine reacted with sodium hypochlorite or potassium permanganate under different conditions and chloramine disinfection were studied. The results showed that with the increase of the amount of preoxidant, pH, polyamine, and Br^- concentration, NDMA production increased correspondingly. Low concentration of NH+₄ could lead to an increase of up to 17.5% in NDMA production, but it gradually decreased to initial level with increased concentration. With the increase of NO-₂ concentration, NDMA production decreased significantly. When the concentration of NO-₂ reached 10 mg/L, the amount of NDMA production was about 15% of the initial level. The preoxidation process of chlorination conforms to the UDMH mechanism, while the preoxidation process of potassium permanganate involves the UDMH mechanism and hydroxyl radical mechanism.