Ankle joint with contracture and/or spasticity can severely disable the mobility and the independence of stroke survivors. Robot-assisted rehabilitation has been proposed to support physicians in providing effective therapies. In this paper, we propose a robot-assisted ankle rehabilitation system integrated with human-computer interaction interface and acquisition of Electromyography signals, joint torque and joint angle. Furthermore, we investigate the effects of proprioceptive neuromuscular facilitation (PNF) rehabilitation method applied in our robotic system. The proposed robot-assisted system has been used in real experiments and provides PNF rehabilitation to five stroke patients for six weeks. Preliminary experimental results suggest that PNF is effective in increasing ankle range of motion(ROM), decreasing ankle resistance torque, and alleviating joint stiffness.