Background: Interleukin 33 is known to have an important influence in the process of myocardial infarction, and the immunoregulatory function of MSCs could be influenced by cell factors. In this study, we evaluated the therapeutic efficacy of IL-33-overexpressing bone marrow mesenchymal stem cells (IL33-MSCs) on myocardial infarction (MI) and detected the inflammatory level and cardiac function in rats. Methods and results: First, we evaluated the proliferation of T cells and polarization of macrophages that had been co-cultured with Vector-MSCs or IL33-MSCs. Co-culture experiments indicated that IL33-MSCs reduced T cell proliferation and enhanced CD206+ macrophage polarization. Second, we determined the inflammation level and cardiac function of PBS-, Vector-MSC-, and IL33-MSC-injected rats. Echocardiography indicated that left ventricular ejection fraction (LVEF) was enhanced in IL33-MSC-injected rats compared with Vector-MSC-injected rats. Postmortem analysis of rat heart tissue showed reduced fibrosis and less inflammation in IL33-MSC-injected rats. Conclusion: These studies indicated that the IL33-MSC injection improved heart function and reduces inflammation in rats with MI compared with PBS or Vector-MSC injections. IL-33 overexpression enhances the immunomodulatory function and therapeutic effects of MSCs on acute MI via enhancing the polarization of macrophages toward M2, enhancing the differentiation of CD4+ T cells toward CD4+IL4+Th2 cells, and finally, reducing heart inflammation and enhancing heart function.Keywords: IL-33; MSCs; Macrophage cell; Myocardial infarction; T cell.