A model simulation of fine particulate matter PM2.5 is conducted in the Houston area using the EPA's Models-3 Community Multiscale Air Quality model (CMAQ). The mass concentrations of PM2.5 and major chemical constituents are calculated and compared with available field measurements. The predicted daily average PM2.5 mass concentrations are consistent with the observed values. The diurnal patterns of PM2.5 mass concentrations are similar throughout the region, with a strong morning peak and a weak peak in the late afternoon to the early evening. High primary emissions, high formation rates of the secondary organic aerosols, and low PBL heights contribute to the morning peak. The major components of the fine particulate matter in this region are sulfate, ammonium, organic carbon and elemental carbon. The balance of the primary cations and anions indicates that fine particulate matter in this region is acidic. Comparisons with field observations reveal that CMAQ produces good simulations of elemental carbon and ammonium, but underpredicts organic carbon, sulfate and nitrate. The CB-4 mechanism, aerosol chemistry and algorithm in CMAQ, and the uncertainty of aerosol precursor emissions are the likely factors responsible for the underprediction of organic carbon, sulfate and nitrate.