Abstract Dicer is the ribonuclease III for synthesis of mature functional microRNAs (miRNAs), which play an important role in regulating cell development. In the mouse ovary, the Dicer1 protein was expressed in both oocyte and granulosa cells of the follicle. In the present study, the role of miRNAs in mouse ovarian development was explored by using Dicer1 conditional knockout (cKO) mouse ovarian tissue ( Amhr2 Cre/-; Dicer flox/flox), in which Dicer1 is deleted specifically in follicular granulosa cells. The morphology and gene expression profile of cKO and wild type (WT) mouse ovaries at various stages of development (day 4, day 8, 8 weeks and 8 months) were examined. Comparative analysis of the follicle number indicated that conditional inactivation of Dicer1 in the follicular granulosa cells led to an increased primordial follicle pool endowment, accelerated early follicle recruitment and more degenerate follicles in the cKO ovaries. In addition, significant differences were noted in the expression of some follicle development-related genes between cKO and WT mouse ovaries, such as Amh, Inhba, Cyp17a1, Cyp19a1, Zps, Gdf9 and Bmp15, suggesting the function of miRNAs in regulating gene expression is time- and gene-dependent. With the Dicer1 inactivation, mmu-mir-503, a miRNA that is more abundant in mouse ovary than in other tissues, was down-regulated significantly. Meanwhile, the expression of mmu-mir-503 decreased notably with follicle development in the gonadotropin-primed mouse ovary. Up-regulation of mmu-mir-503 in primary cultured granulosa cells resulted in the decreased expression of both the target gene and non-target gene at the transcriptional level, which involve genes related to granulosa cell proliferation and luteinization. In conclusion, Dicer1 plays important roles in follicular cell development through the differential regulation of gene expression.