RNA interference (RNAi) is a natural mechanism in organisms in resistance to virus invasion and inhibition of transposon mobility by double-stranded RNA (dsRNA). dsRNA can match with homologous mRNA by base paring to make specific gene inactivation. RNAi was observed in many model organisms, such as Arabidopsis, C.elegans and fungi. Latest study shows that 21～25 nt small interference RNA (siRNA) can mediate specific gene silencing in mammal cells. Being effective and highly specific, RNAi probably becomes a novel technique in knocking gene down and plays important roles in gene function study and gene therapy of diseases.