Objective To explore the role of tendon synovial sheaths in tendon regeneration in vivo. Methods Thirty-six Roman chicken were randomly divided into Group A and B, with 18 chicken in each group. In Group A, the synovial sheaths of the deep flexor tendons in the left middle toes were separated from the up, right and down side without cutting off the tendons themselves. The allograft decellularized tendons were coated with synovial sheaths which were detached partly and fixed on the left side of the normal deep flexor tendons in the middle toes of the left foot. In Group B, the allograft decellularized tendons were directly implanted on the left side of the deep flexor tendons without coating of synovial sheaths. The normal deep flexor tendons from the right foot were used as the control group. The maximum loads and elastic modulus of the tendons at 4th, 8th and 12th week were obtained by mechanical testing, and HE staining was conducted to observe histological changes of the tendons. Results The maximum load at 8th and 12th week and elastic modulus at 4th, 8th and 12th week in Groups A were greater than those in Group B, with significant differences (P<0.05). Group A showed more densely deposited matrices and longitudinally aligned collagen fibers than Group B, and inflammatory cells and fibrous tissues could hardly be found in Group A. In Group B, the collagen fibers were decreased gradually, with disordered alignment. Furthermore, more inflammatory cells infiltration and hyperplasia of fibrous tissues were found in Group B. Conclusions The synovial sheaths can contribute to tendon regeneration, indicating that a proper environment in vivo plays an important role in the engineered tendons. This study has a positive effect on finding proper tendon replacements for patients with tendon deficiency.