Atmospheric oxidizing capacity is the essential feature of urban and regional air. And OH and HO2 radicals are the key species indicating atmospheric oxidizing capacity. Using Guangzhou City as a case, this work has conducted field measurements of photochemistry relevant pollutants including O3, NOx, VOCs, H2O2, HNO2 and CO, SO2. The concentrations of OH radical are measured simultaneously by impregnated filter trapping and HPLC (IFT- HPLC) method. The factors influencing OH levels are assessed. Based on understanding of OH and HO2 air chemistry, the production and removal rates of these 2 radicals are calculated. The results show that the budget of OH and HO2 can generally be closed, the radical transformation between OH and HO2 dominates the sources and sinks of them, and also the photolysis of HNO2 and HCHO is the significant source of OH and HO2 respectively.