【Objective】To observe the levels of CD11b expressions in the surface of neutrophils in serum，and its correlation with left atrial diameter ，and to study the inflammatory mechanisms in atrial fibrillation（AF） patients. 【Methods】Clinical characteristics and blood samples of AF group and sinal rhythm group were collected. CD11b levels in the surface of neutrophils were examined by flow cytometry. Left atrial diameter was examined by echocardiography. 【Results】The AF group included 85 patients ：36 with paroxysmal AF；26 with persistent AF；23 with permanent AF. The sinal rhythm group includes 57 patients. PMN- CD11b levels were significantly higher in Af group than in sinal rhythm group（P＜0.01）. The left atrial diameter was larger in AF group than in sinal rhythm group（P＜0.01）. Multivariate analysis revealed that PMN-CD11b，left atrial diameter and Valvular heart disease were independent predictors of atrial fibrillation.【Conclusions】PMN- CD11b levels were elevated in paroxysmal AF when AF was present and in persistent， permanent AF patients，implying atrial fibrillation was closely related to inflammation；PMN-CD11b were correlated with left atrial diameter，inflammation might participate in the atrial structural remodeling in AF patients ；PMN-CD11b levels were elevated in AF patients with high risk thrombosis，inflammation might have some value to embolic risk stratification according to CHA2DS2-VASc score.