microRNAs (miRNAs) are small noncoding RNAs that mediate RNA interference to suppress protein expression at the translational level. Accumulated evidence indicates that miRNAs play critical roles in various biological processes and disease development, including autoimmune diseases. Invariant natural killer T (iNKT) cells are an unusual CD1d-restricted subset of thymus-derived T cells that are potent regulators of diverse immune responses. Our previous studies with the mouse model of bone marrow-specific Dicer deletion suggest the involvement of Dicer-dependent miRNAs in the development and function of iNKT cells. In the present study, to further dissect the functional levels of Dicer-dependent miRNAs in regulating iNKT cell development, we generated a mouse model with the Dicer deletion in the thymus. Our data indicate that lack of miRNAs following the deletion of Dicer in the thymus severely interrupted the development and maturation of iNKT cells in the thymus and significantly decreased the number of iNKT cells in the peripheral immune organs. miRNA-deficient peripheral iNKT cells display profound defects in activation and cytokine production upon α-galactosylceramide (α-GalCer) stimulation. Our results demonstrate a critical role of the miRNA-dependent pathway in the thymus in the regulation of iNKT cell development and function.Cellular & Molecular Immunology advance online publication, 20 September 2010; doi:10.1038/cmi.2010.49.