Abstract: 【Objective】The purpose of this study was to reveal the histo-cytic characteristics of pecan(Carya illinoinensis)during the process of graft union. 【Method】One-year-old scions of the ‘Pawnee' cultivar were grafted on the two-year-old seedlings of ‘Mahan' by cut grafting, and a histo-cytological observation was performed during the process of graft union. 【Result】Four periods in the process of graft union was observed, including the isolation period of scion and rootstock(1-9 d), callus growth period(9-20 d), cambium ring formation period(20-25 d), and new vascular tissue differentiation period(25-40 d). The callus bridge and cambium ring were formed one month after grafting, which was assumed to be the critical period of graft survival. The union cannot be completed in the dead grafts, in which callus cells were less and browning.【Conclusion】Large amounts of callus occurred in scions earlier than that in rootstocks and the leaves unfolded at the stage when the large amounts of callus occurred; the callus mainly formed in cambium; the inhibition of the growth of callus and the interruption of the process of graft union may be the important factor influencing grafting survival. To improve grafting techniques and grafting survival rate, it appears important that the conditions need to be explored to promote the growth of the callus and the formation of cambium ring.