Recently Hybrid Electric Buses have been widely used in China. In order to study their emissions reduction effects, the exhaust emissions of an in-use diesel-electric hybrid bus have been evaluated both over China City Bus Cycles (CCBC) on chassis dynamometer and on the road using Portable Emissions Measurement Systems (PEMS). The diesel-electric hybrid bus adopts a coaxial parallel mode hybrid system and is driven by electric motor at 0~20km/h while engine keeps idling. Only if the vehicle speed exceeds 20km/h, engine and electric motor will provide driving force together, which leads to more violent transient conditions of engine. Firstly, exhaust emissions of the diesel-electric hybrid bus and a Chinese V stage bus with traditional powertrain have been tested on heavy chassis dynamometer over CCBC. It shows that emission factors of NOx, Particulate Number, Particulate Mass and THC for the diesel-electric hybrid bus are significantly decreased from those of the diesel bus by 47.3%、98.2%、95.6% and 26.2% respectively, but the CO emission factor is higher than the diesel bus because of the more violent transient conditions of engine. Then the Real Driving Emissions (RDE) of the diesel-electric hybrid bus are compared with the dynamometer test results. NOx emission of RDE decreases because of the smaller idling ratio, but PM emission increases because of the relatively larger high speed ratio, and also CO emission increases because of the more violent acceleration and deceleration. To sum up, exhaust emissions of the diesel-electric hybrid bus have been better controlled comparing with diesel bus, but CO emission becomes worse because the transient conditions of engine gets more violent, besides the reduction of NOx has been restricted by the increased ratio of engine idling.