Abstract: By evaluat ing MM5(the Fifth Generation Penn State/ NCAR Meso scale Model) and RADM(Regional Acid Deposition Model), a coupled regional photo chemical model is developed to study tropospheric photo chemical process. The spatial and temporal distributions and variations of regional tropo spheric photochemical precursors, tracers and products such as SO2, NO2, O3, PAN, H2SO4, and HNO3 in northern China are studied with this coupled model. Based on this, the relative roles of horizontal advection, vertical diffusion and deposition as well as chemical transfo rmation in the phot ochemical processes are analyzed. The results show that horizontal advection and vertical diffusion as well as deposition play an important role in influencing concentration distribution and its variation of photo chemical precursors, tracers and products.