Autoimmune diseases are defined as specific, adapted immune reactions against self antigens. Immune mechanism deficiency is a causal factor for B-cell lymphoma in primary Sjogren's syndrome and autoimmune thyroid disease. Thyroid mucosa-associated lymphoid tissue (MALT) lymphoma is a 67-fold and parotid lymphoma is a 44-fold increased risk in Sjogren's syndrome and thyroiditis. MALT lymphoma was not reported in rheumatoid arthritis. We herein report the case of a 56-year-old woman with maltoma of thyroid in rheumatoid arthritis patient.