Abstract: To reduce urban air pollution, more and more low-emission transit buses have been deployed in cities in China. To assess the effectiveness of the adoption of China Ⅳ buses on emissions reductions, real-world pollutant emissions of four China Ⅳ buses with SCR (selective catalytic reduction) technology, together with one China Ⅲ bus, were measured and analyzed in this study. PEMS (portable emission measurement system) was used for the measurement. The results demonstrate that China Ⅳ buses with SCR had 60.8% lower CO emissions, 94.2% lower THC emissions, and 86.2% lower PM emissions compared with the China Ⅲ bus. However, because of the lower conversion efficiency of the SCR systems as a result of a low exhaust temperatures in real-world operating conditions, three of the China Ⅳ buses emitted higher NO_x than the China Ⅲ bus; their NO_x emissions were 187.3%, 229.7% and 157.3% greater than the limit value of the current China Ⅳ standard. Among the China Ⅳ buses, the hybrid bus emitted 42.9% less CO₂,8.8% less CO, and 89.5% less PM. The hybrid bus encountered higher NO_x issues as well. Monitoring of the real-world operating conditions showed that the buses were operating at an engine speed higher than A-speed (1478r/min) only 12.7% of the time. However, the percentage is as high as 80.5% in the ETC testing procedure. The discrepancy between certification cycles like the ETC and real-world operation leads to high NO_x emissions under real-world operating conditions. It was concluded that more effective supervision of heavy-duty vehicle emissions should take more real-world operating conditions into consideration.