This study investigates the formation of 14N- and 15N-nitrogenous disinfection by-product formation upon 15N-chloramination from four dissolved organic matters (DOMs). A series of XAD resins were used to fractionate DOM based on their hydrophobicity and functional group. The results show that hydrophobic acid (HPOA) fraction was the most important precursor pool for haloacetonitrile (HAN); these precursors mainly generated HAN through chloramine incorporation. HPOA and hydrophobic neutral (HPON) gave higher trichloronitromethane (TCNM) yields than the other fractions did. The nitrogen origin upon chloramination of HPOA was primarily from dissolved organic nitrogen (DON). By contrast, chloramines provided the main nitrogen sources during chloraminating HPON and HPOA with low DON to dissolved organic carbon ratios. The results of formation kinetics show that dichloroacetonitrile (DCAN) of which nitrogen source originate from DON formed faster than that of which nitrogen source originate from chloramines. Under normal chloramine exposure (4,000 mg-min/L, equal to 2 mg/L chloramines with around 2 days' reaction duration), 14N-DCAN concentration was two to over five times higher than that of 15N-DCAN. This study also uses a model to calculate the formation concentration of nitrogenous disinfection by-products without hydrolysis effects. The results show that 15N-DCAN formation was linearly correlated with chloramine exposure.