Black carbon (BC), carbon monoxide (CO), and particle number size distribution were measured near a major urban expressway of Beijing during summer and winter field campaigns in 2009. BC was also observed at urban and rural sites. The temporal variations of BC and its relationships with CO and particle number size distribution were analyzed. The average BC concentrations at the roadside site were 12.3 and 17.9 μg m−3 during the summer and winter campaigns, respectively. BC concentrations ranked in the order of roadside > urban > rural. A general diurnal pattern at all sites showed that the higher BC levels were observed at night. The diurnal pattern of summertime BC at the roadside site followed the variations of heavy-duty diesel vehicles (HDDVs). The increased proportion of HDDVs at night contributed to high ΔBC/ΔCO ratios. This study suggests that HDDVs are an important contributor to nighttime BC and particle number concentrations of both Aitken and accumulation modes near major roadways in Beijing, especially in summer. •We first report the temporal variations of BC near roadway in Beijing.•A general diurnal pattern of BC shows the higher levels at night.•Diurnal cycle of BC follows the pattern of heavy-duty diesel vehicles (HDDVs) in summer.•Increased fractions of HDDVs contribute to the higher ΔBC/ΔCO ratios at night.•HDDVs are an important contributor to nighttime particle number concentration.