On-road emission profiles of ultra-fine particle matter with aerodynamic diameter less than 1 μm (PM1) were obtained from a Euro IV diesel bus in Beijing under two typical load mass conditions. PM1 mass and number emissions were recorded second by second using an electrical low pressure impactor (ELPI). An on-road driving mode binning method was applied to relate instantaneous emission rate with real-time on-road driving conditions and an engine operating mode binning method was used to explore the impacts of engine load on PM1 emission characteristics. Our results show that higher load mass will increase both particle mass and number emissions. Measured PM1 mass emission factors were 0.147 g/km and 0.185 g/km under low and high load mass conditions, respectively. PM1 number emission factors were 3.71×1013 #/km and 4.58×1013 #/km. In addition, load mass also influenced the size-resolved concentration of PM mass and number from that sampled bus. For example, under low load mass conditions, an obvious peak of PM number concentration was observed situated in the 2nd channel of the ELPI corresponding to the median diameter 63 nm. By contrast, peak concentration of the PM number was increased under high load mass conditions and situated in the 3rd channel of the ELPI corresponding to the median diameter 109 nm.