<b><i>Background/Aims:</i></b> MicroRNAs (miRNAs) regulate the expressions of cancer-related genes, and are involved in the development and progression of various human cancers. Here, we performed further analyses to determine whether let-7d is functionally linked to Jab1 in breast cancer. <b><i>Methods:</i></b> In situ hybridization and immunohistochemical analyses were used to determine the level of let-7d and Jab1 in breast cancer clinical specimens and its correlation with clinicopathological data. Let-7d overexpressing breast cancer cell lines combined with mouse models bearing cell-derived xenografts were used to assess the functional role of let-7d both <i>in vitro</i> and <i>in vivo</i>. <b><i>Results:</i></b> In this study, we found that let-7d was downregulated in breast cancer tissues, coupled with the elevations of Jab1 protein expressions, compared with paired adjacent noncancerous breast tissues. Let-7d overexpression significantly suppressed the proliferation and invasion in MCF-7 and MDA-MB-231 cells. Dual luciferase reporter assay indicated that Jab1 was the direct target of let-7d. Stepwise studies from <i>in vitro</i> and <i>in vivo</i> experiments indicated that let-7d overexpression inhibited cell growth and decreased Jab1 expressions in breast cancer cells and nude mice tumor tissues. Statistical analyses demonstrated that breast cancer patients with low levels of let-7d or high levels of Jab1 had a significant correlation with worse prognosis. <b><i>Conclusion:</i></b> These findings provide novel insights into molecular mechanism of let-7d and Jab1 in tumor development and progression of breast cancer, and thus let-7d/Jab1 are novel potential therapeutic targets for breast cancer patients
