Ankle joint with spasticity and/or contracture can severely affect mobility and independence of stroke survivors. Due to that, the Achilles tendon(AT) is affected. In this paper, we aim to study changes of AT properties via proprioceptive neuromuscular facilitation (PNF) treatment. A robotic ankle-foot rehabilitation system has been proposed, which consists of a robotic ankle-foot platform and a graphic user interface. In this pilot study, two post-stroke patients participated and carried out a 12-week PNF treatment with the robotic system. The treatment is evaluated quantitatively in AT properties. The evaluation shows that after the PNF treatment, the average decrease of AT length is 4.1 mm (6.5%) and the recovery ratio is 30.4%, while the thickness has no change. The results indicate that the PNF based robotic rehabilitation for ankle joints with spasticity and/or contracture is effective to improve the ankle spasticity/contracture.