This article is focused on increasing the tracking performance of a developed ankle rehabilitation robot subject to external disturbance. A plug-in-type repetitive controller cascaded to a proportional-integral-derivative controller is designed and implemented in order to make improvement in the tracking ability of the parallel mechanism while performing the common range of motion exercises which are intensive training exercises in ankle rehabilitation. First, the trajectory tracking control is simply implemented by the PID controllers, the parameters of which are optimally tuned using Cuckoo search algorithm. Then, the designed RC is plugged into the system and trajectory tracking control is carried out. Performance measurements of the PID controller and plug-in RC controller are estimated using error-based performance methods and considerable improvements are observed in attenuating the external disturbance and decreasing the tracking error when the plug-in RC is implemented.