A 3-D Lagrangian particle dispersion model including the photochemical reaction has been developed and implemented to the data obtained from the southern oxidants study field campaign to examine the capability of the present model. The model includes parameterizations of the entrainment process and the intermixing process between the superposed Lagrangian particles. It is found that the present model is able to simulate quite well characteristic features of the photochemistry for a plume emitted from the isolated major source of the Cumberland Power Plant in Tennessee, USA including the width of plume and the distribution Of O-3, NOy, and nitrate concentration across the plume. This result suggests that the present model can be used more practically in air quality assessment where photochemical reactions are important.