Objective: To investigate the changes in bone metabolic markers and bone mineral density (BMD) among breast cancer patients during the early stage of aromatase inhibitor (AI) adjuvant treatment. Methods: Postmenopausal patients with hormone receptor positive breast cancer, aged from 55 to 65 years, who had completed surgical treatment and/or adjuvant radiotherapy or chemotherapy, and was scheduled to receive AI adjuvant treatment were selected. Before and after AI treatment for 6 months, the serum levels of the bone formation marker procollagen type 1 N-terminal propeptide (PINP) and N-terminal osteocalcin (N-MID) and bone resorption marker β C-terminal cross-linked telopeptides of type I collagen (β-CTx) were detected, and the BMD (T-score) of lumbar spine and hip were also determined. Results: A total of 113 patients were enrolled with an average age of 58.9 years. In the entire group of patients, the PINP level was increased above the normal reference range before treatment, and was further elevated after treatment compared with the pre-treatment level (P<0.05); the N-MID level showed no significant change before and after AI treatment (P>0.05); the post-treatment β-CTx level was significantly higher than that before treatment (P<0.05); the T-scores of lumbar spine and hip had appeared as negative before treatment, and the negativities were further increased, but reached no statistical significance versus those before treatment (both P>0.05). After AI treatment for 6 months, the T-scores of both of lumbar spine and hip, except the correlation between T-score of hip and N-MID did not reach a statistical significance (P>0.05), and showed a significant negative correlation with the levels of each bone formation and bone resorption marker (all P<0.05). Conclusion: There is a mild bone loss in postmenopausal breast cancer patients at 55 to 65 years of age, and the bone turnover activity is further enhanced after AI treatment for 6 months, so the bone-forming agents and antiresorptive therapy should be considered.