Abstract: The increase of oil content in the ripening seeds of tung tree (Aleurites fordii) was negatively correlated with water content, but water stress greatly affected the synthesis of triacylgly cerols and fatty acids. From 25 August to 15 September, the oil content of the seeds rose by 23.87% and the water content fell by 24.55%. The elaeostearic acid content also rose while the palmitic, stearic, oleic, linoleic and linolenic acid contents decreased. Elaeostearic acid comprised 79.42% of the total fatty acids. The most active period for synthesis, accumulation and transformation of fatty acids was in the first 10 days of August and the first 10 days of September, during that time, water supply was critical.An obvious difference was obtained from the comparisons among the compositions of fatty acid of 33 varieties. More differences (about 16.06%) appeared in the percentage of elaeostearic acid. 15 varieties of them had their elaeostearic acid more than 76%. They are all belong to A. fordii. 12 varieties of them have few flowers and single fruit. The contents of this acid in all varities of A. montana were less than 70%. The quality of oil in A. fordii was better than that of A. montana. The varieties of the type with few flowers and single fruit had more content of elaeostearic acid, and less contents of linoeic and oleic acid.