A new found chlorinated disinfection by-product (DBP) in drinking water was isolated and characterized by MS, FTIR, 1 H and 13 C NMR spectroscopy. Single crystal X-ray diffraction method was also carried out to determinate the exact structure of the compound. The crystal is of monoclinic, and space group P2 1 /m with a=7.8800(16), b=6.7950(14), c=8.8350(18)Å, β=115.02(3)°, V=428.67(15)Å 3 , Z=4, D c =1.778g/cm 3 , μ=1.028mm −1 and F(000)=228, R=0.0510 and wR=0.2205 for 982 unique reflections with 918 observed ones [I>2σ(I)]. The results confirmed the structure of this compound. It was finally identified as 2,2,4-trichloro-5-methoxy-cyclopent-4-ene-1,3-dione (TCMCD).