Atmospheric photochemistry of O3-NOX-RH were considered theoretically, to clarify the reasons for the different trends of between the formation of photochemical oxidants (OX) and its primary pollutants for the Low- and High-NOX regimes. Equations of OH, HO2, and production of ozone (O3) as a function of nitrogen oxides (NOX) and reactive hydrocarbons (RH) were represented in this study. For the Low-NOX regime, HO2 radical is proportional to RH but independent of NOX. OH radical is proportional to NOX but inversely-proportional to RH. O3 production is proportional to NOX but has a weak dependence on RH. For the High-NOX regime, OH and HO2 radicals concentrations and O3 production are proportional to RH but inversely-proportional to NOX. In addition, the Osaka Bay and surrounding areas of Japan were evaluated with the mass balance of odd-hydrogen radicals (Odd-H) using CBM-Ⅳ photochemical mechanism, in order to distinguish the Low- and High-NOX regimes. The Harima area (emission ratio, RH/NOX = 6.1) was classified to the Low-NOX regime. The Hanshin area (RH/NOX = 3.5) and Osaka area (RH/NOX = 4.3) were classified to the High-NOX regime.