Rheumatoid arthritis (RA) is the most prominent systemic autoimmune rheumatic disease that may affect different tissues and organs, although the synovial joints represent the major target, The aim of this study is to assess the effect of BMI on rheumatoid arthritis disease activity and evaluate the differnce between clinical and ultrasound assessment of the Disease Activity Score in 28 joints (ΔDAS28) in 3 BMI subgroups according to the WHO classification. Methods: A cross-sectional study was conducted on 52 randomly selected, age & sex matched RA patients diagnosed according to ACR/EULAR 2010 classification criteria of RA, All patients was subjected to full history taking, complete clinical examination, laboratory assessment and Musculoskeletal Ultrasonography. Results and conclusion: Effect of BMI on the disease activity of RA; clinically: there were no significant difference in DAS28 between the normal weight, overweight, and obese patients. But there was a significant difference in swollen joint count between the three BMI groups. Comparison between clinical assessment and musculoskeletal ultrasonography in the three BMI groups ; Musculoskeletal ultrasound  is better than clinical examination to detect synovitis in all  BMI groups. There were significant differences between swollen joint count and grey scale in normal weight, overweight and obese patients. P value = 0.025,  < 0.001 and 0.003 respectively. Musculoskeletal ultrasound is better than clinical assessment to detect the disease activity score in 28 joints (DAS 28) in overweight and obese patients.