At present, medical experts and researchers turn their attention towards using robotic devices to facilitate human limb rehabilitation. An exoskeleton is such a robotic device, which is used to perform rehabilitation, motion assistance and power augmentation tasks. For effective operation, it is supposed to follow the structure and the motion of the natural human limb. This paper propose a robotic rehabilitation exoskeleton with novel shoulder joint actuation mechanism with a moving center of glenohumeral (CGH) joint. The proposed exoskeleton has four active degrees of freedom (DOFs), namely; shoulder flexion/extension, abduction/adduction, pronation/supination (external/internal rotation), and elbow flexion/extension. In addition to those motions mentioned above, three passive DOFs had been introduced to the shoulder joint mechanism in order to provide allowance for the scapular motion of the shoulder. The novel mechanism allows the movement of CGH - joint in two planes; namely frontal plane during shoulder abduction/adduction and transverse plane during flexion/extension. The displacement of the CGH - joint axis was measured experimentally. These results are then incorporated into the novel mechanism, which takes into account the natural movement characteristics of the human shoulder joint. It is intended to reduce excessive stress on patient's upper limb while carrying out rehabilitation exercises.