Ambient particulate matter (PM) and ozone are critical to human health and well-being given their association with increased respiratory and cardiovascular mortality. For urban office buildings in big cities, the use of economizers have been dramatically increased recently for energy saving and ventilation purposes. The objective of this paper is to examine outdoor-indoor transports of ozone and PM_2.5 (particles smaller than 2.5 micron in diameter) for urban office buildings utilizing economizer operating modes. This study employs multi-zone contaminant transport model (CONTAM) for the prediction of outdoor-indoor pollutant dynamics in two cities: Los Angeles and Beijing. The model simulates infiltration of ambient ozone and PM_2.5 into a DOE reference building (medium office) based on outdoor climate condition, outdoor intake, and filtration efficiency. Seasonal variations are considered to capture the influences of high ozone levels during the summer in LA and elevated particle concentrations during the spring season in Beijing. The study results show that outdoor-indoor pollutant transport varies significantly according to the ambient concentration, outdoor weather, season, and ventilation conditions. The indoor concentrations of ozone and particles in both cities are often higher than US EPA and WHO standards, implying that buildings are a critical domain of air pollution control in big cities for reduction of human exposure to ambient air pollutants. The results of this study suggest that economizer flow rate and filtration efficiencies have significant influence on particulate matter transport and indoor concentrations. Comparisons between representative cities provide insights into particle entry into building under various weather and operation scenarios and strategies to maintain good indoor air quality in big polluted cities.