Vehicle generated particle emissions represent a major source of air pollution in urban environments. Recent studies show that monitoring particulate matter in the ultrafine particle (UF) size range (diameter < 100 nm) is critical for assessing adverse health effects. A consensus is emerging that particle number concentration, rather than particle mass, may constitute a better predictor of health effects resulting from exposure to particulate matter (Oberdorster et al., 1990; Pekkanen et al., 1997; Peters et al., 1997; Laden et al., 2000). In this study, a water-based condensation particle counter (WCPC) was used to measure particle number concentrations at a busy intersection of Lai Chi Kok Road and Nathan Lane, located in the center of the urban mega city Mong Kok of Kowloon, Hong Kong. Individual particle numbers and traffic patterns revealed that spikes in the particle number concentration coincided with vehicle acceleration. The highest average particle count (~9.0 × 104 particle/cm3) was observed in an area next to the Hong Kong Environmental Protection Department measurement station (Site A), followed by ~5.5 × 104 particle/cm3 measured at the roadside walkway at the Pioneer Shopping Center (Site B), and ~4.5 × 104 particle/cm3 in front of the SKH Kei Wing primary school (Site C). The highest particle counts occurred when vehicles accelerate, after stopping at a signal light or a bus stop; a peak concentration of 5.4 × 105 particle/cm3 was observed during acceleration of a heavy-duty diesel bus. Peaks with particle number similar to this were reported for a Los Angeles freeway which has the highest percentage (25%) of diesel vehicles (Zhu et al., 2007).