To better understand N-nitrosodimethylamine (NDMA) formation when the ammonia nitrogen was coexist with organic nitrogen compounds, 50mg/L ammonia nitrogen was reacted with dimethylamine (DMA) during the disinfection process. Different reaction conditions such as reaction time, chloramines concentration, pH value and chlorine-nitrogen ratio (free chlorine to ammonia mass ratio) were researched. Furthermore, 3 kinds of representative organic nitrogen compounds (urea, glycine, phenylalanine) in waste water were investigated. The influences of organic nitrogen compounds on chloramines formation and NDMA formation were studied. It was suggested that the pH value and chlorine-nitrogen ratio were significant to NDMA formation. The peak point of monochloramine concentration which was generated by disinfection process moved to a higher chlorine-nitrogen ratio as the addition of the three organic nitrogen compounds. And the NDMA formation at the same chlorine-nitrogen ratio decreased as the addition of three organic nitrogen compounds.