Bangkok Metropolitan Region (BMR) is a region consisting of the populated central area of Bangkok and surrounding provinces, sharing industry, infrastructure, and housing. Due to rapid economic growth, urbanization, and motorization, BMR has suffered serious air pollution issues. Information on major contributing sources of air pollutants such as PM and O3 is required to design appropriate air pollution countermeasures. The Thai government operates a national monitoring network of gaseous and particulate pollutants and precipitation. Previous studies focused on PM pollution in BMR with detailed PM compositions for source apportionment studies. However, there is a lack of long-term monitoring data on PM and precipitation, which are necessary to analyze PM source apportionment. The two joint projects between Japan and Thailand have implemented monitoring of chemical components of PM2.5, coarse particles (>2.5 μm) and precipitation at the selected sites in BMR. Using these monitoring data and receptor models, vehicle exhaust, biomass burning, and secondary formation could be identified as the main sources of PM2.5 in BMR. These results can be compared with the simulation results by the air quality models and will provide information on the sources for which intensive measures should be taken. The projects proposed policy action plans based on air pollution countermeasures described in the national agenda and technical action plans based on the results of monitoring and modeling analysis.