Abstract: To observe the relationship between endogenous hormone and fruit drop of Newhall Navel Orange，the changes of gibberellin（GA），auxin（IAA），zeatin riboside（ZR）and abscisic acid（ABA）contents in navel oranges were assayed after spraying GA₃ and 2,4-D during on-tree storage. The results showed that plant regulators could effectively reduce the fruits abscission through decreasing the GA，IAA and ZR contents and increasing ABA and the ratio of ABA to total contents of GA，IAA and ZR. And the most effective treatment was achieved when a mixture of 20 mg · L^-1 GA3 and 20 mg · L^-1 2,4-D was applied. The fruits were able to remain of good quality after 60 days，on-tree preservation. The results indicated that a combination of descending endogenous GA，IAA，ZR and decreasing ABA and ratio of ABA to total contents of GA，IAA and ZR accelerated the production of the apolexis，thus delayed the ripening and dropping of navel orange during on-tree storage.

Key words: navel orange, plant growth regulator, on-tree storage, endogenous hormone, fruit dropping