Background:Rheumatoid arthritis is a chronic inflammatory disease with uncertain etiology. It is characterized by symmetric polyarthritis in peripheral joints. Its diagnosis is based on clinical findings and serologic tests. However, its diagnosis is rarely conclusive in early course of the disease. So, its early diagnosis could be difficult. The present study was designed to evaluate the role of anti -RA33; an auto-antibody against RA33 in early diagnosis of the disease.Materials and Methods:forty three patients with RA who had been visited in a rheumatology clinic were randomly selected. Their disease has been diagnosed by a rheumatologist. They served as the case group. 55 persons were also chosen from healthy individuals who had attended in other clinic. They served as control. Their age and sex were matched with the case group. Anti-RA33 and RF titers were measured in their blood sample using standard methods.Findings:RF and anti-RA33 titers had significant correlation in the case group (p=0.015). Anti -RA33 test had 98% sensitivity, 20% specificity, 50% positive predictive value, and 90% negative predictive value.Conclusion:Anti -RA33 could have diagnostic and prognostic importance in diagnosis and evaluation of patients with RA, and its differentiation from other small joint disorders, particularly when the other serologic tests are negative.