Objective To determine the expression level of microRNA-21 (miR-21) in serum exosomes of patients with different severities of asthma, and to explore the diagnostic value. Methods A total of 82 patients with asthma who did not received any treatment and 80 healthy control volunteers were enrolled in this study. The asthma cases were divided into four groups according to the severity of asthma, including intermittent group (n=20), mild sustained group (n=22), moderate sustained group (n=22), and severe sustained group (n=18). Expression levels of miR-21 were determined using qPCR and the differences were compared among the five groups. Spearman correlation analysis was used to analyze the correlation between miR-21 expression level and severity of asthma. The receiver operating characteristic (ROC) curve was used to evaluate the diagnosis performance of miR-21 expression level in serum exosomes for different severities of asthma. Results The expression levels of miR-21 were significantly higher in the intermittent, mild sustained, moderate sustained and severe sustained groups than that in the healthy control group (all P<0.01). The difference of miR-21 expression level was statistically significant between the intermittent, mild sustained, moderate sustained and severe sustained groups (all P<0.01). Spearman correlation analysis showed that the relative expression level of miR-21 in serum exosomes was positively correlated with the severity of asthma (r=0.974, P=0.016 7). The areas under ROC curve of exosome miR-21 for diagnosing intermittent, mild sustained, moderate sustained and severe sustained asthma were 0.657, 0.769, 0.847 and 0.916, respectively (all P<0.01). Conclusion The expression level of miR-21 in serum exosomes is effective in diagnosing different severities of asthma, and miR-21 in serum exosomes may be a new non-invasive biomarkers.