|Purpose: We investigated the incidence and time of dislocation and other factors associated with dislocation of bipolar hemiarthroplasty related to the treatment of femoral neck fracture in old age patients. Materials and Methods: Between January 2002 and April 2014, 498 femoral neck fractures (467 patients) were treated with bipolar hemiarthroplasty and included in this study. All surgeries were performed using the posterolateral approach. The incidence of dislocation was investigated. A comparative analysis between a control group and dislocation group was performed with respect to patient factors including age, gender, body mass index, comorbidities, the ASA (American Society of Anesthesiologists) score, mental status and center-edge angle, and surgical factors including type of femoral stem, leg length discrepancy, femoral offset and method of short external rotator (SER) reconstruction. Results: The incidence of dislocation was 3.8%, and the dislocation occurred on average 2.2 months (range, 0.6- 6.5 months) after operation. No difference in patient-related factors was observed between the two groups. However, a smaller center edge (CE) angle was observed in the dislocation group (42.1。±3.2。vs. 46.9。±5.4。, P<0.001), and significantly lower incidence of dislocation was observed in tendon to bone repair group (0.7% vs 7.8%, P<0.001). Conclusion: SERs should be repaired using the tendon-to-bone repair method to reduce dislocation rate in elderly patients who undergo bipolar hemiarthroplasty using the postero-lateral approach due to femoral neck fracture. In addition, patients with smaller CE angle should be carefully monitored due to high incidence of dislocation.