RNA interference (RNAi) is a potent and highly specific gene-silencing phenomenon which is initiated or triggered by double-stranded RNAs (dsRNAs). Shortly after the development of RNAi, small interfering RNAs (siRNAs) that are 21 nucleotides in length with a 3' nucleotide overhang were shown to be very effective in mammalian cells. Much effort has been dedicated to the application of siRNAs, both as biological tools and as therapeutic agents. Currently, synthetic siRNA would be the method of choice for clinical purposes. However, natural RNA strands are quickly degraded in biological fluids. Chemically synthesized unnatural nucleotides have been developed and introduced into the siRNA strand. For example, modification of the ribose moiety with a 2'-deoxy, 2'-O-methyl, or 2'-fluoro group, or modification of the phosphate backbone have been examined. Although these modifications improve the stability of siRNA in serum, they often cause a decrease in RNAi activity. There is also concern that unnatural RNA derivatives are toxic in the human body. A method to stabilize nontoxic natural RNA strands should be very useful for applications in RNAi technology. We came up with an idea that nano-structural design stabilizes natural RNA. We tested several new designs such as dumbbell RNA, double stranded circular RNA, or branched RNA in biological stability and RNA interference activity. Consequently, dumbbell or branched design offered prolonged RNAi effect due to high biological stability.