Abstract: Objective：To isolate and identify exosomes from serum samples of the patients with polymyositis / dermatomyositis (PM/DM), and analyze their protein composition preliminarily. Methods：Exosomes from serum samples of the patients with PM/DM were isolated and purified by the ExoQuick^TM kit. The morphological characteristics and particle size of exosomes were determined by transmission electron microscope (TEM) and NanoSight analyzer, respectively. The surface markers of exosomes such as CD9, CD81 and Flotillin2 were identified by western blot. The concentration and composition of exosome protein were determined by the BCA method and SDSPAGE, respectively. Results：The exosomes from serum samples of PM/DM patients displayed round or oval vesicles with membrane structure under TEM, and their diameter range was about (92±67) nm. western blot showed that these exosomes expressed CD9, CD81 and Flotillin-2. The total protein concentrations of exosomes in the patients with PM/DM and healthy controls were 14.68 (6.00,32.55) μg/μL and 14.09 (8.00,23.28) μg/μL, respectively. SDS-PAGE showed that highabundance proteins enriched in 5570 kD in both PM/DM patients and healthy controls, and that there were different bands in 4055 kD between them. Conclusion：Exosomes are isolated from serum samples of the patients with PM/DM successfully, and their protein concentration and composition are analyzed preliminarily, which provides the experimental evidences for further finding differential proteins.