Abstract: After flower induction, with the neighbored body conditions, some longan trees can successfully blossomand yield, while others can only sprout leaf buds. In order to research the original reasons which caused the differences, we took off -season flowering trees and non -flowering ones as contrastive materials after flower forcing, and studied the changes of carbon content in the climax leaves, tender leaves, climax shoots, tender shoots and terminal buds of longan tree. The results showed that carbon played a major role in flowering process. Moreover, these trees could easily flower when the carbon content in climax leaf reached 50.93 mg/g, but when the carbon content was below 37.40 mg/g in climax leaf, the longan trees were difficult to flower. At the same time, the maturity of tender leaf also had a great influence upon flowering. In comparison, a mature leaf contained more carbon content than that of a tender one, and was easier to flower.This can be used as a new judgment standard of flower induction in off-season longan trees.