Based on data collected from January 1996 to May 2000, this paper outlines a general climatology of the noontime maximum ultra-violet index (UVI) in Taiwan. In summer, high UVI values of 14~15 can be observed at a 2400m-high mountain site and at southern and eastern coastal sites. It is noted that Taipei, a polluted city, has only a 19% chance of observing a UVI over 9 in July, while Hualien, a clean coastal city, has an 83% chance. In addition, total ozone has a minor spatial variation pattern and is not the major factor affecting the change in surface UVB flux. Flux transmissivity, which is estimated by dividing the observed UVI with a model-estimated value for a clear sky condition, has a positive correlation coefficient of 0.73 ~ 0.84 with the UVI, which supports the idea that clouds and local pollution strongly affect the surface UVB flux. A UVI forecast system is developed based on the findings stated above. Forecast of total ozone is simplified, while subjective and objective cloud-effect adjustment processes are emphasized. The root-mean-square difference between the forecast and observed UVI is about 1.7~2.9, with a correlation coefficient of 0.6~0.76 and a bias of -0.54~0.5. The worst forecast is at Alishan, the mountain site, because the local fast-changing weather is difficult to forecast.