With the purpose of analyzing uptake and translocation of fertilizer N in Satsuma mandarin, ¹⁵N-labelled ammonium sulfate was applied as a fertilizer to experiment tree of about 20 years old in the following four seasons each: i.e. before harvest in autumn, after harvest in autumn, spring and summer. The translocation of ¹⁵N to leaves tool place within the year of application when it was supplied before harvest in autumn, whereas ¹⁵N applied before harvest in the autumnal season was translocated only at almost the same time with that applied in the following spring season. The translocation rate of applied ¹⁵N to leaf organs in the tree plants reached a peak about 25 days after fertilization when the soil temperature was about 10℃ of higher. The greatest contribution or applied ¹⁵N to fruit organs was observed under the summer N-fertilization. However, the summer application distributed more ¹⁵N to peel tissue than to flesh in the late autumn. These results were effectively incorporated in the improvement of cultivation practices of Satsuma mandarin in various parts of Japan, particularly in rationalizing fertilizer application in production centers.