Very efficient ligation of oligodeoxyribonucleotides was attained through a simple molecular construct, which is composed of one stem and two branches (Y-shape), with use of T4 RNA ligase. Single-stranded DNAs (naturally, RNAs also) of more than 100 nucleotides (even 800 nts) were considerably ligated, approximately as theoretically expected. Owing to the molecular construct adopted, such a tiny amount of ligation products could be amplified to a sufficient amount by PCR and then recovered as single-stranded DNAs. This advantage of being amplifiable is shown to be useful for both combinatorial chemistry and evolutionary molecular engineering, which deal with a pool of diversity molecules.