Abstract: Pot culture experiment was conducted with acid yellow-brown earth to study the effects of molybdenum application on fatty acids and mesophyll cell ultrastructure of winter wheat leaves at late tiller stage. Among four main fatty acids, octadecadienoic acid and octadecatrienoic acid content of winter wheat leaves were increased dramatically while octadecenoic acid content was decreased by Mo application. There was no difference in hexadecanoic acid content between Mo-sufficiency and Mo-deficiency plant. Through the electron microscope,some differences between Mo-sufficiency and Mo-deficiency plant in the cell or the organelle were observed. The result showed as follows: Applied with molybdenum, the completeness of organelle(chloroplast,chondriosome,cell nucleus,etc.), and the stability of the cell membrane of the Mo-treated winter wheat leaves were better than those of Mo-deficiency plant; molybdenum application was beneficial to the stability and the completeness of mesophyll cell ultrastructure of winter wheat leaves.