Purpose: Osteoarthritis is a progressive disease and the most common form of joint inflammation. Moreover, it is the most common cause of functional disability in the elderly. Among the multiple and predisposing factors influencing the disease are demographic indicators and occupational factors. The present study aimed to investigate the relationship between age, pain severity, Body Mass Index (BMI), occupation, and educational level, and the severity of functional disability in patients with Knee Osteoarthritis (KO). Methods: This descriptive study was performed on 97 KO patients referring to the Novin private physiotherapy clinic of Semnan University of Medical Sciences from April to March 2017. The study participants were selected through a simple nonprobability sampling technique. Literate individuals with the educational level of guidance school and above and diagnosed with KO were included in the study. Individuals with a history of inflammatory arthritis diseases, such as rheumatoid arthritis, soft tissue rheumatoid arthritis, fibromyalgia syndrome, bursitis, tendonitis, the neurological and vascular conditions of the lower extremity, mental problems, and malignancy were excluded from this research. The data related to the variables such as age, gender, occupation, history of osteoarthritis, pain intensity, involved side, educational level, and the Knee Injury and Osteoarthritis Outcome Score (KOOS) for knee disability were collected by a self-report questionnaire. This study was approved by the Research Ethics Committee of the University of Social Welfare and Rehabilitation Sciences. Using SPSS, Spearman’s correlation coefficients were calculated to examine the relationship between the study variables. Results: Spearman’s correlation coefficients revealed no significant correlation between the KOOS scores and age, occupation, and educational level. The obtained results suggested a poor significant association between KOOS and BMI; however, there was a strong significant correlation between KOOS and pain intensity Visual Analogue Scale (VAS) (P<0.05). Conclusion: The severity of functional disability in patients with KO based on KOOS questionnaire scores, was well correlated with pain severity, but poorly associated with age and BMI. The obtained data indicated no significant relationship between disability and occupation and educational level.