Abstract: The fertilizing technique has been studied with the ¹⁵N tracing method, the material is seedlings of Frexinus pennsylvanica. The result show that the applying of fertilizer of seedlings is different in four growing stage and five position of fertilizing, the applying of fertilizer is high during the fast-growing period, meanwhile the position of fertilizing should be in the area of 5cm diameter around the root in seedling stage, 5—10cm diameter in fast-growth period. The depth should be 0—5cm in seedling stage and 5 —10cm in fast-growing period respectively.