The Thousand-Island Lake was formed by dam construction that resulted in habitat fragmentation, that is, the formation of islands surrounded by water. We sampled trees and shrubs on 50 islands in the lake to examine relationships between island areas and species diversity with various curve fitting methods. Our results showed that logarithmic functions, power functions and S-type curves adequately described the relationship between island area and plant (trees, shrubs, and woody species) species richness, with logarithmic functions being the optimal model. S-type curves and inverse functions depicted relationships between Shannon-Wiener index of trees and woody plants and island area reasonably well, but were not good depictions of relationships between Shannon-Wiener index of shrubs and island area. On islands less than 1 hm2, the best species-area models were S-type curves and inverse functions for Shannon-Wiener index of trees and woody species; the same applies to Shannon-Wiener index of shrubs on islands larger than 1 hm2. The relationships between Simpson index or Pielou index of trees, shrubs, and woody plants and island area were not significant. Species diversity index increased rapidly with island size for small islands, but much more slowly for large islands. For all plants, the turning point of the rate of increase in species richness was about 4 hm2. For trees and woody species, the turning point of the rate of increase in Shannon-Wiener index was about 1 hm2, and was about 0.15-0.2 hm2 when only islands less than1 hm2 were considered.