Background: Total knee arthroplasty (TKA) is among the most frequent orthopedic procedures. TKA has been shown to provide significant benefits to patients with rheumatoid arthritis (RA) and osteoarthritis (OA). We hypothesized that patients undergoing primary TKA with diverse preoperative diseases would have varying expectations for both pain and function following TKA. Materials and Methods: Patients diagnosed with OA or RA participated in this prospective cohort investigation. We assessed variables including Knee society score (KSS), and patient’s quality of life using 36-Item Short Form Survey Instrument (SF-36). Pain was also measured using the Visual Analogue Scale (VAS). Results: In this study, 606 patients including 412 females and 194 males with a mean age of 65. 5 ± 8. 7 years were included in the study. Among patients, 46 patients underwent TKA due to RA and 560 patients due to knee OA. There was no statistically significant difference in term of KSS clinical score before the operation (P = 0. 101). The mean of KSS clinical score and functional score variables at 6 months and 12 months after the operation were higher in OA group in comparison to RA group. SF-36 was significantly improved in OA group in comparison to RA group. Also, VAS in the RA group is significantly higher than in the OA group after 12 months of follow-up. Conclusion: Our results demonstrate that patients with OA after TKA have better KSS clinical and functional results than patients with RA.