Purpose. We examined the role of miR-20b in development of thymoma-associated myasthenia gravis, especially in T cell proliferation and activation. Materials and Methods. Using qRT-PCR, we assessed expression levels of miR-20b and its target genes in cultured cells and patient samples and examined the proliferation of cultured cells, using MTT cell proliferation assays and flow cytometry based cell cycle analysis. Activation of T cells was determined by both flow cytometry and qRT-PCR of activation-specific marker genes. Results. Expression of miR-20b was downregulated in samples of thymoma tissues and serum from patients with thymoma-associated myasthenia gravis. In addition, T cell proliferation and activation were inhibited by ectopic overexpression of miR-20b, which led to increased T cell proliferation and activation. NFAT5 and CAMTA1 were identified as targets of miR-20b. Expression levels of NFAT5 and CAMTA1 were inhibited by miR-20b expression in cultured cells, and the expression levels of miR-20b and NFAT5/CAMTA1 were inversely correlated in patients with thymoma-associated myasthenia gravis. Conclusion. miR-20b acts as a tumor suppressor in the development of thymoma and thymoma-associated myasthenia gravis. The tumor suppressive function of miR-20b in thymoma could be due to its inhibition of NFAT signaling by repression of NFAT5 and CAMTA1 expression.