The anatomical and histological development of the graft union in splice grafting in hazelnut (Corylus avellana L.) was examined. One-year suckers were used as rootstock and scion materials. Grafting was done manually in the first week of December. The grafted plants were kept for 28 days at 26-28 °C and 70-80% relative humidity conditions. The cross and longitudinal sections with a thickness of 12-20 microns taken from the graft union area by a rotary microtome 14, 18, 21, 26, 32, 52 ve 140 days after grafting were examined microscopically. Early callus proliferation from rootstock and particularly scion two weeks after grafting was generally slow. The initial cambial differentiations in callus tissues were seen in sections 18 days after grafting. Cambial continuity between rootstock and scion was established 32 days after grafting. The sections in the following periods exhibited that the graft partners were in vascular relationship. The development of the graft union was successfull with all its stages. It was observed that the amount of callus tissue proliferated during the first two weeks after grafting directed the subsequent development of the union. In this respect, it is thought that developing methods that encourage callus formation in the early periods of the union will increase the success in related studies. The anatomical and histological examinations revealed that rootstock and scion thicknesses that are very close to each other and accordingly well matching of graft partners from cambial zones affect the development of union positively and prepare a suitable basis for early vascular differentiation.