Through determining the relative rate of conductance of the cutting leaves of 27 species of street trees and data processing by computer, the semi-fatal heat temperature, the zero﹒heat temperature and the temperature of collapsing point is obtained. The results showed that thirteen species (Platanus ×acerifolia , Cinnamomum camnphora, etc. ) are thermostable trees, twelve species (Sophora japonica, Liriodendron chinense× tulipifera, etc. ) have a middle heat hardiness, while Magnolia denudata and Cornus macrophylla are weak on the heat hardiness.