Abstract: Accordingly to the reactions of letins of three species of nodulated leguminous trees: Albizzia julibrissin, Sophora davidii, Robinia pseudoacacia and three species of unnodulated leguminous trees, which are Cassia bicapsularis, C. corymbosa, C. suf fruticosa with 13 species of rhizobium,the letins of three species of nodulated leguminous trees can react with most of rhizobium, However, some un-nodulated leguminous trees for example C. suffruticosa also can recognize rhizobium but can not be nodulated at last. So we think that the activity of letins of leguminous trees with rhizobium is absorbed in the root’s surface of leguminous trees, and that is the pre-condition of founding the relationship of symbiotic nitrogen fixation.