A novel 2′,4′-BNA/LNA analog bridged by guanidine, termed as guanidine bridged nucleic acid (GuNA), was synthesized and incorporated into oligonucleotides. Thermal stabilities and nuclease resistance of GuNA-modified oligonucleotides were investigated and compared with those of 2′,4′-BNA/LNA and natural DNA oligonucleotides. GuNA exhibited interestingly high binding affinity towards complementary ssDNA than 2′,4′-BNA/LNA.