Abstract: The reaction of nalidixic acid with the free available chlorine was investigated. The reaction proceeded rapidly in the presence of excess free chlorine，and three types of products including mono-, di- and tri-chloro products were observed. A major product was isolated and characterized by EI mass spectrometry, high resolution mass spectrometry, nuclear magnetic resonance spectroscopy (NMR). The chlorination products proposed in present study were not reported before except that of resulted from the replacement of carboxyl substituent with chlorine.