Abstract This study aims to investigate the influence of micrometeorology on atmospheric visibility in Taipei Basin by carrying on routine and intensive visibility observation, analyzing particle concentrations and size distribution, monitoring atmospheric optical coefficients of particles, and collecting meteorological conditions (wind direction, wind speed, temperature and humidity). Based on the information of the broad view of the meteorological parameters from 1998 to 2005, the air pollution episode in Taipei Basin mainly occurred during the winter periods accompanied with the weather patterns of high pressure outflow style I, high pressure outflow style II or circus-sluice of high pressure outflow. Based on the collected visibility data from Taipei of Central Weather Bureau for the past 21 years (1983~2004), it shows the visibility increasing each year. The visibility seasonal variation was also observed higher than that during winter. The mean visibilities from Tamsui, Songshan, and Sindian directions are 10.54 km , 9.72 km and 8.62 km, respectively. In general, the visibility from the Tamsui direction is slightly higher than those the from two directions, This result shows visibility is influenced not only the pollution factor and meteorological factor, but affected by the local topography. Our study revealed that the visibility data leads on the Tamsui, Songshan obserrative directions is better. Two intensive sampling campaign were conducted during August 15~22 of 2004 and January 15~22 of 2005 on the roof of Taipei County Government, respectively. The results showed mass concentration of particulate matter in winter is higher than that in summer. A bi-mode size distribution of aerosol particles was also observed in the Taipei basin. Two peaks of aerodynamic diameter of PM were observed at 0.56~1.0 µm and 3.2~5.6 µm, respectively. In addition, extinction coefficient is always high during winter. Mean scattering coefficient is about 80~108 Mm-1 while the mean absorption coefficient is around 9~35 Mm-1, during the 1st intensive sampling period. Mean scattering coefficient is about 101~119 Mm-1 while the mean absorption coefficient is around 16~64 Mm-1, during the 2nd intensive sampling period. Our study also shows that visibility has a higher correlation with fine particles(R=0.70~0.96) than that with coarse particles(R=0.045~0.629). Hance, visibility is affected mainly by fine particles. In general, the intensive observation of visibility also shows that in summer the visibility is higher than that in winter. In addition, the analysis from MM5 shows that the visibility for the Tamsui direction is usually influenced by the prevailing wine direction, It is because of that northwestern from sea brings lots of water spray into the basin. This spray water will increase the particle diameter and cause the increasing of extinction coefficient. It will also impair the visibility.