To investigate the photodegradation process of dimethyl phthalate (DMP) in the ultraviolet light (UV)-H2O2 system, mass spectrometer was used to identify degradation products and further more, to speculate the possible mechanisms of degradation process. Experimental results showed that the degradation efficiency of 10 mg x L(-1) DMP reached 92.3% in 90 minutes in the UV-H2O2 system, and the more H2O2 was added, the faster DMP decomposed. The pH of DMP solution decreased from the initial 6.5 to 4.98, because of the generating of organic phenolic acids and small acid molecules generated by the degradation of DMP. By the GC/MS as well as LC/MS analysis, degradation products of the DMP in the UV-H2O2 system were thought to comprise six categories, from which we could infer that hydrolysis occurs simultaneously on the two side chains of the DMP, generating phthalic acid that can quickly change to the more stable terephthalic acid. In addition, DMP could also occur benzene ring replacement as well as the ring condensed of the side chains. Finally, by the role of *OH, DMP and its aromatic intermediates occurred ring-opening reaction, and benzene ring was destroyed and generated organic acids of small molecular, which was further mineralized to CO2 and water.