PURPOSE: To evaluate the osseointegration of titanium alloy cortical screws with the passage of time. MATERIALS AND METHODS: Fifty four titanium alloy cortical screws (24 mm in length, 3.5 mm in diameter) were implanted bilaterally in the tibial diaphysis of adult mongrel male dogs of similar size and weight (30 ±5 kg). The insertion torques, radiographs, undecalcified histology, histomorphometric analysis and extraction torques were evaluated at 2, 4 and 8 weeks after surgery. RESULTS: The extraction torque at 2 weeks (1.14±0.470 cN. m) was significantly lower than the insertion torque (1.76±0.609 cN. m) (p=0.0071), the extraction torque at 4 weeks (2.57±1.36 cN. m) was slightly improved and the extraction torque at 8 weeks (3.18±0.499 cN. m) was significantly higher than insertion torque (p=0.0005). Direct bony contact in the early phase was poor and intervening fibrous tissue was observed at the bone-screw interface. However, the fixation between the bone and the screws improved with time. The percentage of bone-screw contact at 8 weeks (33.1±18.5%) was higher than that of 2 weeks (22.4±12.9%), but not statistically significant. CONCLUSION: Because of thermal injury or pressure necrosis, the fixation strength of titanium alloy cortical screws at 2 weeks after implantation is significantly lower than that at the insertion time. So, we should keep in mind the initial phase weakness of screw fixation when we allow the patients the range of motion exercise or weight bearing and the improvement of the initial phase fixation is very important in clinical results.