Synthetic biology has emergied as a promising research area that enables researchers to simplify complicated biological systems into managable pieces and to redesign brandnew biological circuits by assembling the simplied biological modules recruited from different living organisms. Several approaches have been investigated to provide logical connectivity between well-defined binding events among relevant biomolecules and the way how the biomolecualr interactions lead to programmable circuit behaviors. These include engineering of biological circuits like gene regulatory systems, metabolic fluxes and signal transduction pathways. Here we present a novel synthetic riboregulatory system that is switchable by small chamicals. The riboswitch shows more than ten-fold decrease in translational efficiency of a reporter gene by addition of small chemicals. Sequence design of the riboswitch is easily achieved to fine-tune mRNA secondary structures and the consequent switch behaviors including target specificity and sensitivity. Owing to the good sequence designability, the riboswitch seems promising for biomedical applications to high-throughput screening systems, biosensors and in vivo bioimaging.