To research the role of the Dicer and miRNA in vascular smooth muscle cell (VSMC), the conditional gene targeting method, sequential biopsy pregnancy mice, histopathology, immunofluorescence, PCR, Western blot and Real time PCR were used to examine carefully the vasculopathy, the changes of Dicer, miRNAs and proteins of signal transduction pathway in conditional knock out the Dicer (Dicer cKO) embryos. Results indicated that three types of mice with different genotype that was wild type mice, heterozygotic type mice and homozygotic type mice were generated during breeding Dicer cKO mice. In them, wild type mice and heterozygotic mice were able to born, and no evident clinical abnormal showed when they were born, but homozygotic mice was not able to born and died in mother's abdomen. The development delay of Dicer cKO embryos was found in embryonic growth 12.5days (E12.5), the obvious vessels expansion, blood stasis and far-ranging diffuse hemorrhage in E14.5 and death in E15.5. The lesions of Dicer cKO embryos' vessel walls appeared as early as at E13.5, mainly showed irregular arrangement and lower proliferative cells; the pathologic changes of vessel walls thinning, collapse and irregular lumen appeared at E14.5. All destroyed structure of vessel wall, few cell proliferation, damaged barrier function of vessel wall, permeability enhancement and exudation of red blood cell occured at E15.5. The expression of VSMC marker genes, most detected miRNAs and phosphorylated signal transduction pathway proteins that were extracellular signal-regulated kinase and protein kinase showed distinct down regulation at E14.5. This experiment proves that the Dicer is necessary gene for vessel development. The Dicer regulates the expression of VSMC marker genes through commanding miRNA generation and maturation, so as to promote proliferation and differentiation of VSMC and guarantee the integrity of vessel wall structure.