This research developed a device that can be attached to the conventional Knee-Ankle-Foot-Orthosis for controlling the knee in individuals after stroke. The device automatically assists the flexion and extension movements of the knee joint at the appropriate timing. We aimed to clarify the effect of this device on gait performance, kinematic pattern and muscle activation during gait. Seventeen participants after stroke were recruited in this study. They walked with this device for 3 minutes, and we measured the 10m gait time, number of steps, knee motion using electric goniometer and muscle activities of lower limbs during gait. The gait speed, number of steps, and range of knee joint improved by using the device with assist, and those changes remained without assist. For muscle activities measured by electromyography in the lower limbs, although the device only assists one side of the limb, it was able to alter EMG of both paretic and non-paretic limbs. By assisting only the paretic knee movements, the device was able to improve the performance, kinematic pattern and muscle activation during gait, which remained as an aftereffect.