Urban aerosol concentrations in the size range of $0.01 sim 1.0 mum$ have been measured by using an electrical aerosol analyzer from May through October, 1984. The total diurnal variation of the number concentration indicates that a minimum value is observed at 3 hr and a sharp increase is noticed early in the morning with a subsequent slow and continuous increase from around 7 hr until 20 hr. After that it is decreased to reach its minimum by dawn. However, both surface and volume concentrations have shown that their first maxima at 8 hr and their second at about 20 hr simultaneously. It is found that the aerosol number is mainly governed by the particles in the size range of $0.01 sim 0.1 mum$, while most volume is in $0.1 sim 1.0 mum$ size range. It is known fact that particles of $0.1 sim 1.0 mum$ size range affect the visibility reduction in the atmosphere. The monthly variation of aerosol concentration remarks its minimum in summer. The main factors influencing the aerosol concentration are emission of autoexhausts, various processes of production and removal, and meteorological parameters.