Patients with hemiplegic cerebral palsy often have noticeably deviant arm posture and decreased arm movement. Here we develop a comprehensive assessment method for the upper extremity during walking. Arm posture score (APS), deviation of shoulder flexion/extension, shoulder abduction/adduction, elbow flexion/extension and wrist flexion/extension were calculated from three-dimensional gait analysis. The APS is the root mean square deviation from normal, similar to Baker's Gait Profile Score (GPS) [1]. The total range of motion (ROM) was defined as the difference between the maximum and minimum position in the gait cycle for each variable. The arm symmetry, arm posture index (API) was calculated by dividing the APS on the hemiplegic side by that on the non-involved side, and the range of motion index (ROMI) by dividing the ROM on the hemiplegic side by that on the non-involved side. Using the APS, two groups were defined. Group 1 had minor deviations, with an APS under 9.0 and a mean of 6.0(95% CI 5.0-7.0). Group 2 had more pronounced deviations, with an APS over 9.0 and a mean of 13.1 (CI 10.8-15.5) (p = 0.000). Total ROM was 60.6 in group 1 and 46.2 in group 2 (p = 0.031). API was 0.89 in group 1 and 1.70 in group 2 (p < 0.001). ROMI was 1.15 in group 1 and 0.69 in group 2 (p = 0.003). APS describes the amount of deviation, ROM provides additional information on movement pattern and the indices the symmetry. These comprehensive objective and dynamic measurements of upper extremity abnormality can be useful in following natural progression, evaluating treatment and making prognoses in several categories of patients.