Graft compatibility is a complex mechanism with a wide range of different physiological, biochemical, and anatomical interactions. In the present paper, features of the graft union on the combination of the European chestnut (Castanea sativa Mill.) on an oak (Quercus vulcanica Boiss.) rootstock based on whip grafting was investigated. The trial was conducted in the greenhouse at the Faculty of Agriculture, Adnan Menderes University, in western Turkey during 2006-7. The grafts were made with scion from chestnut genotype N-3-4 on an oak rootstock. Cross and longitudinal sections of the graft union for examination were taken 30, 60, 90, 120, 150 and 210 days after grafting, and fixed in 70% ethanol. Microscopic observation of the anatomical and histo-cytological structure of the graft union area revealed that new cambium, xylem, and phloem tissues were formed in the samples 120 days after grafting. The results showed that, graft compatibility exists between chestnut and oaks. Callus formation and differentiation of vascular tissues started 150 days after grafting.