Real-world emission data are collected from heavy-duty diesel buses using the Portable Emission Measurement System (PEMS), and driving activity data are collected from diesel transit buses using the Global Positioning System (GPS) in Beijing. The collected emission data are used to establish emission rates for Vehicle Specific Power (VSP)-bins, as well as to validate the VSP-based emission estimation approach for diesel transit buses. The collected driving activity data are used to identify the VSP distributions of diesel transit buses, which are combined with the established emission rates for VSP-bins to analyze the emission characteristics of diesel transit buses. It is found that, spatially, the diesel transit bus emissions decrease from the city center to the outbound direction gradually; the highest emissions occur on minor arterial roads and the lowest emissions occur on expressways; and emissions at bus stations are much higher than those on the regular road segments. Temporally, the two emission peaks occur during the morning and evening rush hours. It is also found from the analysis that the public transit priority policy can not only improve the operational efficiency of diesel transit buses, but reduce their exhaust emissions as well.