Retinoic acid-inducible gene I (RIG-I) is a cytosolic receptor of viral RNA and recognizes double-stranded viral RNAs (dsRNAs) containing two or three 5’ phosphates. Critical role of viral RNAs is recognizing viral RNAs and induces type I interferons. A few reports of 5’- PPP-independent RIG-I agonists have emerged, but little is known about the molecular principles underlying their recognition. We recently found that the bent duplex RNA from the influenza A panhandle promoter activates RIG-I even in the absence of a 5’-triphosphate moiety. Here, we report that noncanonical synthetic RNA oligonucleotides containing G-U wobble base pairs that form a bent helix can exert RIG-I-mediated antiviral and anti-tumor effects in a sequence- and site-dependent manner. We also implies that structural dynamics of synthetic RNAs conserving G-U wobble base pairs by using NMR spectroscopy. We present synthetic RNAs that have been systematically modified to enhance their efficacy and outline the basic principles for engineering RIG-I agonists applicable to immunotherapy.