Abstract: The introduction of dsRNA into cells can interfere with the expression of an endogenous gene homologous to the dsRNA, which results in the loss_of_function phenotype .This response is called RNA interference (RNAi ). Genome sequencing projects generate a wealth of information , however, the ultimate goal of such projects is to accelerate the identification of biological function of genes. This creates a need to fill the gap between sequence and function . Here we summarized the mechanism of RNAi and its application. The mechanism of RNAi has been brought to light by the identification of RISC complex and an enzyme named Dicer. As a reverse genetic tool , RNAi can be used to isolate loss_of_function or reduction_of_function mutants and to identify the biological function of genes . Therefore, RNAi can be applied for functional genomic analysis of animals and plants on a large scale.