Abstract: Nitrogenous disinfection byproducts (N-DBP) in drinking water attract more and more attention, among which chloraldimines are one kind of off-flavor N-DBP generated from different kinds of amino acids, posing the general chemical structure as R—CH=NCl. Based on GC/MS and HPLC/MS, chloroaldmines originated from valine and phenylalanine were identified. The other unknown by products of phenylalanine were identified by GC/MS and the transformation pathway of phenylalanine was proposed. The results show that chlorination of valine (Val) can induce isobutyrochloraldimine, while phenylalanine (Phe) can induce chlorophenylacetaldimine, phenylacetaldehyde and 2, 6-diphenylpyridine. Phe reacts with chlorine to produce chlorophenylalanine and dichlorophenylalanine, which can decompose and transform to phenylacetaldehyde. Dichlorophenylalanine can also produce N-chlorophenylacetaldimine. HPLC coupled with liquid-liquid extraction was emplyed to purify chlorophenylacetaldimine, and GC/MS was used to determine the concentration. Occurrence of chlorophenylacetaldimine was measured, and the concentration ranged from 5.03-11.26 μg/L in four drinking water treatment plants of a certain city. Results show that the contamination risk of chloroaldmine exists in drinking water.