Gait rehabilitation is a necessary training process during early-stage treatment for patients suffering from stroke or Spinal Cord Injury (SCI). However, patients with decreased lower extremity muscle strength may have difficulty keeping the stability of the upper trunk and have the possibility of falling down. Therefore, it is necessary to provide patients with Partial Body Weight Support (PBWS) and to ensure safety during bipedal locomotion. In this paper, we introduce a mechatronic system design of a walking assist robot with Body Weight Support (BWS) mechanism to assist locomotor rehabilitation training for patients with stroke or SCI. The BWS functionality is realized by using a Variable Stiffness Mechanism (VSM) and ground load signals can be measured using a pair of force sensor-based robotic shoe systems. The proposed control system design is implemented in the QNX real-time operation system and the experimental result illustrates the validity of the proposed robotic architecture.