This research was designed to observe the stages of graft union in relation to different growth rate between the rootstock Ottawa 3 and three apple varieties, Smoothee, Summerred and Braeburn. Three to five graft samples of each rootstock/scion combinations were taken at 30 days interval until 12 months passed after from grafting. Samples were fixed in ethanol (70%). Transverse sections in 20-30 mu m thickness were cut with a rotary microtome. Five base stages of graft formation were examined on samples: (1) development and positions of necrotic layers, (2) proliferation of callus cells, (3) formation of callus bridge at the graft interface, (4) cambial continuity and (5) formation of vascular tissues. In the sections prepared from subsequent samples of graft combinations were seen as a good union. Necrotic layers were eliminated in the callus, except for the outer cortex. In outer cortex region, necrotic layers were transformed to bark. Callus filled the gaps between interface of the stock and the scion. Cambial continuity occurred successfully and vascular tissues were connected. Thus, no evidences for tissue incompatibility were found in this study. Histological observations showed a compatible graft union between the graft partners which have different growing characteristics in relation to tissue formation.