Novel nucleoside analogs have been desired for selective formation of non-natural type triplexes containing TA or CG interrupting sites. We have previously reported that the W-shaped nucleic acid (WNA) would be a useful skeleton to develop new base analog for the formation of non-natural triplexes of antiparallel motif. In this study, we have found that triplex forming oligonucleotide (TFO) incorporating the new WNA analog, WNA-beta T, formed more stable triplex than natural triplex with high selectivity to the TA site. It is also noted that the TFO containing WNA-beta T did not form aggregates at the physiological condition of K+.