Exoskeleton robots are wearable electromechanical structures interacting with human limbs. They are used for extending or replacing human performance in power augmentation and rehabilitation applications. The neuromuscular system of the human body provides flexible and stable movement with minimum energy consumption by means of the compliant actuation of human joints. Similar to human body, compliant actuation can be used to maximize the performance in exoskeleton robots. In the present study, we designed a new biomimetic compliant lower limb exoskeleton robot (BioComEx). Firstly, the current exoskeleton robot designs and biomechanics of the human body joints are reviewed. Then, according to the inferences of human joint biomechanics review, ankle joint is designed as variable stiffness actuator; knee and hip joints are designed as series elastic actuators. Kinetostatic analysis of these joint mechanisms is conducted, and finally the design details of each joint and complete exoskeleton structure adapted to a human dummy model are explained.