A sampling campaign was conducted at an urban roadside site in Hong Kong from Nov. to Dec. in 2017 using a suite of state-of-the-art instruments to monitor compositions of non-refractory sub-micron particulate matter (NR-PM 1) and gaseous pollutants. Results showed that the average NR-PM 1 concentration was 26.1±0.7 μg/m 3 (average±95% confidence interval) and organic aerosol (OA) contributed the most to NR-PM 1 with a proportion of 57.7±0.2%. The aerosol size distributions of bulk composition of NR-PM 1 presented a peak at~ 600 nm with internal mixtures of the organic and inorganic components, while there were a larger proportion of primary organic particles at< 200 nm, indicating intensive emissions of primary organics at this site. Positive matrix factorization (PMF) analysis was applied to the measurement data and four OA components were identified, including a hydrocarbon-like OA (HOA) factor, a cooking organic aerosol (COA) factor and two oxygenated OA (OOA) factors of different oxidation levels: less oxidized OOA (LO-OOA) and more oxidized OOA (MO-OOA). Strikingly, the contribution of MO-OOA was the highest (30.9±0.4%), suggesting high oxidation degree and/or high regional background in the roadside environment. Moreover, the proportion of COA reached 25.4±0.3% at this roadside site with heavy traffic fleet, which was even higher than the percentage of HOA (p< 0.01). The average ratio of C 3 H 3 O+/C 3 H 5 O+(2.01±0.01) and the opposite pattern of C 3 H 3 O+/C 3 H 5 O+ to O x during daytime hours suggested that the COA was oxidized to some extent when transported to the site. The findings implied that cooking activities are a significant source of organic aerosols in Hong Kong, even at a busy road. Control measures should focus on both cooking and traffic emissions in Hong Kong.