Background. Knee osteoarthritis is the most common joint disease and one of the leading causes of disability in old age. The present study aims to analyze the stability of standing with and without perturbation in patients with moderate knee osteoarthritis. Methods. Twenty-eight (14 men with knee osteoarthritis and 14 healthy individuals) were evaluated for postural control. In standing tests, the effects of disease (osteoarthritis vs healthy), vision (open vs closed eyes), and support condition (quiet standing vs on the unstable plate) on balance were studied. Results. The results showed that the presence of knee osteoarthritis significantly reduced the root mean square of hip joint flexion in patients compared to the control group (p = .024). Elimination of vision and reducing the base of support by standing on an unstable plate led to local instability in the joints close to the perturbation, especially the ankle (p < .001). Center of pressure data also showed that the mean (p = .034) and variability (p = .003) of the anterior-posterior excursion was significantly higher in patients. Patients with knee osteoarthritis are more vulnerable to falling on an unstable plate. Conclusion. The body used a postural stiffening strategy to prevent falling forward, especially on an unstable plate, and postural adjusting in the medio-lateral direction. In rehabilitative treatments to prevent falls based on the sensory re-organization plans e.g. rocking board, foam standing, game therapy, etc. it may be more efficient to focus on the distal joint muscles.