We designed and synthesized dumbbell-shaped nanocircular RNAs for RNA interference applications, which consist of a stem and two loops(1). RNA dumbbells are specifically recognized and cleaved by the human Dicer enzyme, and are thus transformed into double-stranded RNA in cells, although this RNA is resistant to degradation in serum. The structure was optimized to maximize its RNAi activity. The most potent activity was achieved when the stem length was 23 base pairs. The RNAi activity is prolonged by the shape of the molecule, an endless structure, compared with that of normal siRNA.