STUDY DESIGN: Animal experiment. OBJECTIVE: To evaluate whether the use of polyetheretherketone (PEEK) rods for posterior spinal fixation can improve screw stability. METHODS: Sheep models of anterior-posterior cervical fusion were used in this study. Six sheep were randomly assigned to the PEEK rod group and titanium alloy group. A total of 8 screws and 2 fixing rods were implanted in each sheep. At 24 weeks postoperatively, a computed tomography (CT) evaluation, pull-out test, micro-CT evaluation and histological evaluation were conducted to evaluate screw stability in the harvested surgical segments. RESULT: According to the CT evaluation, there were no signs of screw loosening in either group. The pull-out force and energy of the PEEK rod group were significantly higher than those of the titanium alloy rod group. Denser and thicker trabecular bone around the screw was observed in the PEEK rod group according to the micro-CT reconstructed images, and quantitative analysis of the micro-CT data confirmed this finding. In the histological evaluation, more abundant and denser bone trabeculae were also observed in the PEEK rod group. However, there was no significant difference in the bone-screw interface between the 2 groups. CONCLUSION: Posterior spinal fixation with PEEK rods can increase screw stability by promoting bone growth around the screw but cannot promote bone integration at the bone-screw interface in an animal model study. This finding presents a new idea for clinical practices to reduce screw loosening rate.