This study examines the local ozone photochemistry in the urban air. The photochemical formation and destruction of ozone was modeled using a photochemical box model. For the model prediction of ozone budget, measurements were carried out from an urban monitoring station in Seoul (37.6˚N, 127˚E), Korea for intensive sampling time period (Jun. 1～15, 2003). Photochemical process is likely to play significant role in higher ozone concentrations during the sampling period. The results of model simulation indicated that photochemical ozone production pathway was the reaction of NO with HO2 while ozone destruction was mainly controlled by a photochemical destruction pathway, a reaction of H2O with O(1D). The contribution of NMHCs to formation and destruction of ozone in the urban was significant. This was entirely different from remote marine environment. The rates of net photochemical ozone production ranged from 0.1 to 1.3 ppbv h-1 during the study period.