In this study, the real-world NOx and particle emissions of buses burning pure diesel fuel (D100), biodiesel fuel with 20% blend ratio (B20) and liquefied natural gas (LNG) were measured with portable emission measurement system (PEMS). The measurement conducted at 6 constant speed, which ranged from 10km/h to 60 km/h at 10km/h intervals, and a period of free driving condition. The relationship between vehicle specific power (VSP) and NOx/particle emissions of each bus were analyzed. The results show that the change rules of NOx, PN and PM emission factors with the increase of VSP were basically the same for the same bus, but for the bus using different fuel, the change rules may change. In VSP bin 0, the vehicles were mostly in idle condition and the emission factors of NOx, PN and PM of three buses were all in a relatively high level. In low VSP interval, which ranged from bin 0 to bin 4, the emissions of three buses first decreased and then increased with the growth of VSP. In high VSP interval, namely bin 4 to bin 7, NOx, particle mass and particle number of D100 and B20 buses continuously increased, but those emission factors of LNG bus decreased with the increase of VSP. Although the main part of particle emissions of all three buses were nanoparticles (with aerodynamic diameter smaller than 48.5nm in this study), VSP and fuel had influences on particle size distribution. With the increase of VSP, the proportion of smaller size particle gradually decrease and bigger particles decreased. The particle emissions of D100 and B20 buses had a trimodal distribution, while LNG bus had a unimodal distribution, which were related to the fuel. Comparing to pure-diesel-fueled bus, B20 and LNG buses had a reduction of more than 40% in PM emission factors, but showed a higher NOx and PN emissions.