Dicer1, an RNAse III enzyme, is a key factor for the production of microRNAs involved in post-transcriptional gene silencing. To elucidate the roles of Dicer1 and the microRNA pathway in early embryo development, we initially evaluated its gene expression in mouse oocytes and embryos in vitro. The transcript levels in GV stage oocytes steadily decreased up to the 2-cell embryo stage, and expression remained stable during morulae and blastocyst formation. DICER1 protein synthesis was additionally observed in mouse oocytes and early embryos. Silencing of mRNA expression by RNA interference (siRNA) did not inhibit development up to the blastocyst stage. Real-time RT-PCR experiments confirmed the decreased expression of selected transcription factors, including POU domain, class 5, transcription factor 1 (Pou5f1), SRY-box containing gene 2 (Sox2), and Nanog homeobox (Nanog). Moreover, POU5F1 protein expression was suppressed by Dicer1 siRNA. The results suggest that Dicer1 gene expression is associated with the levels of transcription factors, Pou5f1, Sox2, and Nanog which possibly regulate differentiation processes at the blastocyst stage.