Intra-osseous fixation of transplanted flexor tendons has been used widely in clinical practice, but the anchoring process has not been elucidated well. In the present study, tendon fixation was observed histologically in chickens whose flexors are similar anatomically to human tendon. Right deep flexors of the third digits were cut at the insertion, and then reanchored to the distal phalanx by a pull-out method. Sixteen young chickens (2-3 months after hatcing) were used for the study and sacrificed 4 days to 12 weeks after the procedure. Histological changes at the re-anchored area were examined by light and scanning electron microscopy. A control study confirmed that deep flexor tendons of young chickens directly anchor to the bone tissue, where fine fibrils are dispersed in the bone matrix. Deep flexors re-anchored to the distal phalanx showed a healing process between the tendon and bone. Granulation tissue surrounding the tendon pulled through the anchor hole matured as newly formed connective tissue which connected the tendon and bone, and simultaneously ossification appeared from the bone marrow side. The point of insertion of the re-anchored tendon was made of three layers, consisting of tendon, fibrocartilage and bone, 8 weeks after the operation, similar to the normal anchorage. Ossification seemed to proceed with small granules, about 0.2μ in size, which may correspond to matrix vesicles. It was indicated that firm fixation between tendon and bone is achieved at about 4 weeks when Sharpey's fibers appear histologically.