Small RNA widely exists in one of the main function regulation of gene expression in an organism, rather than direct protein synthesis of RNA. SiRNA and mirnas are important for both small RNA, and play irreplaceable function. Both can inhibit the expression of mRNA, but methods of action vary, siRNA is mainly led by RNAi (RNA interference) to gene silencing; Micrornas in cell proliferation, differentiation and other activities play a key role in regulation. SiRNA and biosynthesis of mirnas are introduced in this paper, siRNA and microRNA mediate gene silencing mechanism, as well as siRNA in yeast, plants, and the human body by triggering the change of the epigenetic modifications regulating gene expression model, the mechanism of siRNA and mirnas is discussed in the end for the influence of disease treatment and drug research and development.