Abstract: Objective To investigate the role of macrophage extracellular traps ( METs) in the pathogenesis of rheumatoid arthritis(RA) by detecting the level of METs in synovium of RA patients. Methods The synovial tissues of the patients with RA and trauma surgery were collected. The inflammatory infiltration in synovium was detected by HE staining. The expression ( F4/80* ) and polarization (F4/80* CD86*, F4/80* CD206* ) of macrophages，and the level of METs formation ( F4/80 H2A+ ) were observed by laser confocal microscope. The levels of citrullinated histone H3 ( CH3) and peptidyl arginine deiminase4 ( PAD4) in synovial cells were detected by Western blotting ( WB). Results Compared with the trauma surgery patients, RA patients showed more severe inflammatory infiltration in synovial tissue and increased inflammatory cells. The total number of macrophages and the proportion of typeI macrophages (M1) were significantly increased ( 98+7.09 vs 10+1.15, 0.78+0.022 vs 0.13+0.011, P<0.001). No significant difference for the proportions of type I macrophages ( M2) was found between the two groups ( 0.23+0.019 vs 0.18+0.012, P>0.05). The formation of METs in RA group was significantly higher than that in control group (0.65+0.023 vs 0.18+0.012, P<0.001 ). Western botting showed that the levels of PAD4 and CH3 in RA group were significantly higher than those in trauma surgery patients ( 0.82+0.018 vs 0.20士0.015, 0.83+0.013 vs 0.40+0.012, P<0.001 ). Conclusion Macrophages in the synovium of RA patients were significantly increased, and M1 type macrophages should be the predominant cells. The formation of METs significantly increased in RA.