This study conducted roadside particulate sampling to measure the total suspended particulate (TSP), PM 1 0 (particles <10μm in aerodynamic diameter) and PM 2 . 5 (particles <2.5μm in aerodynamic diameter) mass concentration in 11 urbanized and densely populated districts in Hong Kong. One hundred and thirty-three samples were obtained to measure the mass concentrations of TSP, PM 1 0 and PM 2 . 5 . According to these results, the TSP, PM 1 0 and PM 2 . 5 mass concentrations varied from 94.85 to 301.63μgm - 3 , 67.67 to 142.68μgm - 3 and 50.01 to 125.12μgm - 3 , respectively. The PM 2 . 5 /PM 1 0 ratio of all samples was 0.82 which ranged from 0.62 to 0.95. The PM levels and PM ratios in metropolitan Hong Kong significantly fluctuated from site-to-site and over time. The PM 2 . 5 mass concentration in different districts corresponding to urban industrial, new town, urban residential and urban commercial were 77.64, 87.50, 106.96 and 88.54μgm - 3 , respectively. The PM 2 . 5 level is high in Hong Kong, and for individual sampling, more than 60% daily measurements exceeded the NAAQS. The mass fraction of PM 2 . 5 in PM 1 0 and TSP is relatively high when compared with overseas studies.