Abstract Background & aim: Rheumatoid arthritis is one of the most widespread and disabling diseases in the society that causes chronic inflammation of the synovial layer and joint destruction. Numerous pharmacological and non-pharmacological therapies are suggested for the treatment of this disease, including radiocinectomy as an effective and safe treatment with beta-particle radiolucent in patients who do not respond to or receive drug therapy. The aim of this study was to determine the histologic evaluation of radionuclide yttrium-90- hydroxyapatite in rheumatoid arthritis induced by rheumatoid arthritis in the rat knee joint. Methods: In the present experimental study, 40 Wistar rats (3 months old) were used by intraperitoneal injection of Enrofloxacin (20mg / kg) for the development of an animal model of rheumatoid arthritis. Subsequently, a dose of 50 microliters of yttrium-90-hydroxyapatite with 100-microcurie activities was injected into the left knee joint of the studied animals.Then, the rate of leakage of yttrium-90 radiotracer after 24 hours and its therapeutic effects were evaluated by histologic findings through specific staining, 2 weeks and 2 months after injection. The collected data were analyzed using Shapiro–Wilk method. Results: Statistically significant changes in parameters such as articular cartilage thickness, chondrocyte count, proteoglycan level, calcium deposition, and knee arthroplasty rate two weeks after 90Y-HA injection showed cellular damage due to particle irradiation. An increase in the thickness of articular cartilage and the number of chondroblasts two months after injection may indicate therapeutic effects of yttrium-90 radiocolloids on the knee joint of rats with rheumatoid arthritis. Conclusion: It seems that radiosynovectomy of yttrium-90-hydroxyapatite may be one of the effective therapeutic modalities in reducing inflammatory factors and improving rheumatoid arthritis induced in rats.