In this work, using an external orthosis, we present a new design for gravity balancing of the human leg in order to decrease the required joints toques during walking. Gravity balancing is often used in industrial machines to decrease the required actuator efforts during motion. In this proposed design, passive gravity balancing orthosis is connected to the human leg on the shank and its other end is fixed to a walking frame. The human leg is assumed to have three DOF for the hip medial and lateral rotation, hip abduction and adduction, hip flexion/extension and one DOF for the flexion/extension at the knee. The external orthosis connected at the shank, together with the human leg, creates a kinematic closed loop. The kinematic loop constraint can be satisfied during walking by choosing appropriate link lengths for the external orthosis. Gravity balancing of the human leg and the external orthosis is achieved by making the potential energy of the combined system, human and the machine, to be configuration invariant. These conditions are satisfied by choosing appropriate inertia parameters of the segments of the orthosis and addition of proper springs. Regarding the possible motions of the human leg (two, three and four degree-of-freedom motion of the human leg in the sagittal, coronal and transverse planes), feasible designs are then presented. The joint torques at the hip and knee joints are derived using inverse dynamics of closed loop system (human leg and external orthosis) or open loop system (human leg without orthosis). The joint torque trajectories of the human leg during walking are computed and compared for the motions with different DOFs for two cases of human leg and the external orthosis and human leg without external orthosis. The results show that the joint torques in all designs with the external orthosis are less than the joint torques of the human leg without the external orthosis at slow walking. The main advantages of the design of human leg with external orthosis as compared to the existing exoskeletons are as follows: (i) This design has a better alignment between the human leg and the orthosis. (ii) We can also get the full extension of the knee with this design. (iii) It is required less hardware such as force-torque sensors between human leg and orthosis to compute the joint torques. However, there are also some drawbacks for this design such as: (a) It increases the inertia of the system, which may be a drawback during fast walking. (b) Modeling of the human leg with the external orthosis, which makes a kinematic closed loop, is more complicated than the modeling of the existing exoskeletons, namely, open loop system. Key Words Rehabilitation Robots, Passive external orthosis, Human joint torques, Gravity balancing.