When the guaranty moisture condition of a region is evaluated, besides the annual or seasonal income of precipitation, one should also take into consideration the expenditure of soil moisture. By this way, it will be more comprehensive than just using precipitation as a factor to evaluate the moisture resource of a place or of different seasons. This paper adopts the ratio of Thornthwaite's potential evaportranspiration and precipitation as the criteria for aridity in order to calculate the aridity of Taiwan: 1. When aridity equals 1, it denotes that the potential evaportranspitation and the precipi-tation are the same, and the climatic environment is good. 2. When aridity is less than 1, i.e., potential evapotranspiration is smaller than precipitation, then there is a humid climate and dry crops generally do not need irrigation, which is only practiced in paddy fields. Drainge should be kept in mind at flat lands. 3. When aridity is more than 1, potential evapotranspiration is larger than precipitation and there will be water deficit. Measures to prevent drought is a necessity. Most part of Taiwan belongs to humid region, with exception in the winter of Southwestern Taiwan where arid region exists. This shows that “aridity” can be a very good criterion.