A regional air quality modeling study was done for a typical summer smog episode that occurred in Northeast of United States and Southern Ontario in July 13-17, 1999. The MM5/SMOKE/CMAQ modeling system was used for this study. Sensitivity tests indicated that the MM5 cloud prediction can significantly impact the photochemical processes, therefore affecting the ground ozone and fine particulate matter levels significantly. The photolysis rates in CMAQ require cloud correction factors through MCIP which is tightly linked to the MM5 cloud prediction . However, appropriate cloud predictions in MM5 rely on the selection of a cumulus parameterization and microphysics condensation scheme that correspond to the MM5 model resolutions, the weather conditions being modeled and the other correlated physics options.