The diurnal variation of air pollution over Metro Manila is studied by using a theoretical model. The model equations consist of a set of fluid dynamical equations and an equation of mass transport for predicting air pollution. The dynamical equations are based on a mixed layer model of the atmosphere. The source of pollutants is assumed to be uniformly distributed over the Metro Manila area. The source strength varies in time in accordance with the variation of vehicular traffic emission during the day. Model simulations are made in order to determine the effect of varying conditions of the prevailing atmosphere. The results of the simulations indicate a predominantly semi-diurnal variation in the concentration. The maxima usually occur in the early morning and in the evening. This theoretical result is in agreement with long-term averaged observations.