Gene silencing can occur either through repression of transcription, termed transcriptional gene silencing (TGS), or through translation repression andmRNA degradation, termed posttranscriptional gene silencing (PTGS). PTGS results from sequence-specific mRNA degradation in the cytoplasm without dramatic changes in transcription of corresponding gene in nucleus. Both TGS and PTGS are used to regulate endogenous genes. Interestingly, mechanisms for gene silencing also protect the genome from transposons and viruses. In this paper, we first review RNAi mechanism and then focus on some of its applications in biomedical research such as treatment for HIV, viral hepatitis, cardiovascular and cerebrovascular diseases, metabolic disease, neurodegenerative disorders and cancer.