In this study, the regional transport and characteristics of PM2.5 pollution were examined through a case study in Beijing, China. The results from an intensive monitoring program indicate that the inorganic particles (sulfate, nitrate, and ammonium), organic carbons, and the elements accounted for 35.5, 24.2, and 15.3% of the total PM2.5 on an annual average basis, respectively. The proportions of such PM2.5 components also showed clear seasonal variations. An integrated MM5-CMAQ modeling system was then developed to examine the regional transport of PM2.5 and its components in Beijing within four typical months of 2010. The results indicate that the annual average total trans-boundary contribution ratio (TBCR) is 42.2, 46.3, 77.4, and 61.6% for the concentrations of PM2.5, sulfate, nitrate, and ammonium, respectively. A logarithmic relationship was found between the total TBCR and the PM2.5 concentrations in Beijing for different seasons. Further investigations showed that trans-boundary transport played a major role in Beijing’s PM2.5 concentrations during the period of high pollution levels, with an annual average TBCR of 54.6%. As a result, the control of PM2.5 pollution in Beijing needs effective cooperation between Beijing and its surrounding regions, especially during periods of heavy pollution.