In this study, we developed an active ankle-foot orthosis (AAFO) which can control the dorsi/ plantarflexion of the ankle joint to prevent foot drop and toe drag during walking. To prevent slapping foot after heel strike, ankle joint has to be controlled actively to minimize forefoot collision with the ground. In the late stance, ankle joint also has to be controlled to provide the toe clearance and help the push-off. 3D gait analyses were performed on five healthy subjects (age: 27.5±2.1 years, height: 169.4±4.3cm, weight: 66.4±2.3kg) using a near-infrared 3D motion analysis system (Vicon 612, VICON, U.S.A.). Three different gait conditions were compared: the normal gait without AFO, the SAFO gait with the conventional plastic AFO, the AAFO gait with the developed AFO. As a result, the developed AAFO could preeminently induce the normal gait compared with SAFO. Additionally, AAFO can prevent the foot drop by proper plantarflexion during loading response and provide enough plantarflexion moment as driving force to walk forward by the sufficient push-off during pre-swing. AAFO also can prevent the toe drag by proper dorsiflexion during swing phase. In addition, SAFO can bring a very inefficient gait by the abnormal pelvic movement with a compensation for the limited movement of ankle joint. On the other hand, the AAFO can induce an efficient gait with the similar movement as in the normal gait. These results indicate that the developed AAFO may have more clinical benefits to treat foot drop and toe drag, compared with conventional AFOs, and also could be useful in polio patients or patients with other orthotic devices.