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Background miR-15b is significantly and consistently downregulated in different clinical phenotypes of myasthenia gravis (MG). However, its role in pathogenesis of MG is still not clear. This study aimed to explore the function of miR-15b in MG. Material and methods Blood samples from early-onset MG, late-onset MG, thymoma patients, and healthy participants were collected. The expression pattern of IL-15 and miR-15b was identified by qRT-PCR and ELISA in patient serum and mouse tissue samples. The regulative role of miR-15b on IL-15 expression was verified in an experimental autoimmune myasthenia gravis (EAMG) mice model. Results qRT-PCR and ELISA showed that miR-15b expression was significantly lower and IL-15 expression was significantly higher in all EMG, LMG, and thymoma cases compared to healthy controls. Based on mouse model, we confirmed that miR-15b knockdown could increase IL-15 expression in healthy mice, while miR-15b overexpression could inhibit IL-15 expression in EAMG mice. Through searching in bioinformatics databases, we identified a highly conserved consequential pairing between IL-15 and miR-15b. Subsequent dual luciferase assay further verified this match. Conclusions This study is the first to report the miR-15b-IL-15 axis can directly regulate IL15 expression, which helps to further explain the abnormal IL-15 expression in MG patients and the pathogenesis of MG.