[Abstract] Objective To investigate the value of ultrasonically measured femoris rectus cross sectional area (RFCSA) in predicting postoperative delirium (POD) in elderly orthopaedic patients. Methods This study was a prospective and observational study. A total of 153 elderly orthopedic patients, aged 65-93 years, regardless of gender, were selected from April 2023 to December 2023, with American Society of Anesthesiologists (ASA) classification gradesⅠ‒Ⅲ. The transversal area of the rectus femoris was measured by preoperative ultrasonography, and POD occurrence was evaluated by Confusion Assessment Method (CAM) scale 1 to 5 days after surgery. Potential indicators of POD were analyzed by Logistic regression, and receiver operating characteristics (ROC) curves were analyzed to evaluate the accuracy of the test. Results Delirium occurred in 27 (18%) of 147 patients, 63% of whom were female. RFCSA of male patients without POD was 2.24±0.32 and 1.33±0.16 in POD group (p 0.05). The RFCSA of women without POD was 2.04±0.24 and 1.15±0.17 in POD group (p 0.05). In multivariate logistic regression analysis, advanced age, low BMI, low RFCSA, and frailty were independent rick factors for POD (adjusted odds ratios of 1.86, 1.78, 1.66, and 3.08, respectively). The area under RFCSA curve was 0.820 for males and 0.879 for females (p 0.05). Conclusion Ultrasonic measurement of the femoris rectus muscle cross-sectional area (RFCSA) has good predictive value for POD. Low RFCSA was associated with POD occurrence in elderly orthopedic patients.