According to ISO9223, classification of atmospheric corrosivity can be performed either by corrosion rate measurement or by environmental classification. In the present study, categories of corrosivity of atmosphere in Taiwan are classified based on environmental classification in terms of time of wetness, pollution of airborne salinity and SO2 guided by ISO9223. It is found that along west part of Taiwan, northward of Daja creek, corrosiveness category is classified as C4 except Taipei city, which is rated as C3; while at southward of Daja creek, corrosiveness category is classified as C3. For coastal and seashore areas, corrosiveness category is classified as C5.