The ultraviolet/persulfate (UV/PS) process was used to degrade dibutyl phthalate (DBP) at different reaction conditions and a steady-state kinetic model was established based on the elementary reactions involved as well. UV/PS process, which can generate both sulfate radical (SO4-center dot) and hydroxyl radical (HO center dot)proved by the evidence that methanol had a larger inhibition impact on the degradation than tert-butyl, could effectively degrade DBP. The second-order rate constant of (1.2 +/- 0.1) x 10(8) M-1 s(-1) between DBP and SO4-center dot and the second-order rate constant of (6.3 +/- 0.1) x 10(9) M-1 s(-1) between DBP and HO center dot were determined by competition kinetics. The degradation efficiency of DBP was affected.by PS dosage, initial DBP concentration, solution pH value, natural organic matter (NOM) and inorganic anions. Increasing PS dosage could enhance the degradation of DBP before PS dosage reaching 1.6 mM. However, the pseudo-first-order rate constants (k(o)) decreased as initial DBP concentration increased probably owing to the radical scavenging effect of DBP. Generally, the k(o) value decreased slightly at pH values ranged from 4.0 to 6.0 while decreased significantly as pH increased from 6.0 to 8.0, which might be due to the scavenging effect on SO4-center dot and HO center dot by OH- and increasing concentration of HPO42- (with higher scavenging capacity than H2PO4-) from the phosphate buffer with rising pH value. NOM and HCO3- showed an inhibition effect on the degradation of DBP through quenching SO4-center dot and HO center dot. However, Cl- had a dual role on the degradation of DBP in the UV/PS system, which enhanced the degradation at lower concentration (0-1 mM) and suppressed as Cl- concentration further increased to 2 mM. Additionally, a simple steady-state kinetic model involving SO4-center dot and HO center dot was developed to simulate the k(o) values and the radical contributions in this reaction system. Generally, HO center dot had a higher contribution to DBP degradation than SO4-center dot The total cost including the electrical energy per order and oxidant cost was increased with increasing PS dosage and the oxidant cost was the major one. The results in this study can serve as a guide and indicate that UV/PS process is feasible for the treatment of water contaminated with DBP.