Spinal orthosis is generally applied to the patients with adolescent idiopathic scoliosis (AIS) during puberty to mechanically support the spine and prevent further deterioration. However, the optimum location of pressure is not easy to be determined and the X-ray taken is not a real time presentation of the spinal curvature. With the advancement of clinical ultrasound, tracing spinal processes along a scoliotic spine becomes possible, which means spinous process angle (SPA) can be obtained from ultrasound images. Moreover, SPA is found to be highly correlated with Cobb's angle. Since the outcome of orthotic intervention for AIS is considered to be associated with accurate orthosis fitting, this study seeks to apply three-dimensional (3-D) ultrasound in the fitting procedure of spinal orthosis for patients with AIS. The accuracy of pressure pad location in brace can help to improve the effectiveness of spinal orthosis treatment. By means of the ultrasound assessments, spinous process angle is examined and used as the parameter to evaluate the optimal location for pressure pad. The intra-rater reliability [ICC (1, 3)] for using ultrasound to measure SPA is >0.9 (p<0.05). Furthermore, the correlation between Cobb's angle estimated from the measurement of SPA in 3-D ultrasound images and Cobb's angle measured from X-ray is highly significant (R=0.98, p<0.01). According to these findings, ultrasound can be further developed as a non-invasive real-time assessment tool for spinal curvature especially in fitting stage to improve the treatment effect of the spinal orthosis.